Merge branch 'mh/fast-import-optimize-current-from'

Often a fast-import stream builds a new commit on top of the
previous commit it built, and it often unconditionally emits a
"from" command to specify the first parent, which can be omitted in
such a case.  This caused fast-import to forget the tree of the
previous commit and then re-read it from scratch, which was
inefficient.  Optimize for this common case.

* mh/fast-import-optimize-current-from:
fast-import: do less work when given "from" matches current branch head

diff --combined fast-import.c
index 9d949d1f77eb5949765a5c6f0adc37b4a0fcb0ef,ece009662379dd26a523cb082b50c11109c96b14..6a0c0ac570169e2f58c41b5c0427aefce1ed18f9
--- 1/fast-import.c
--- 2/fast-import.c
+++ b/fast-import.c
@@@ -134,17 -134,16 +134,17 @@@ Format of STDIN stream
    ts    ::= # time since the epoch in seconds, ascii base10 notation;
    tz    ::= # GIT style timezone;
  
 -     # note: comments, ls and cat requests may appear anywhere
 -     # in the input, except within a data command.  Any form
 -     # of the data command always escapes the related input
 -     # from comment processing.
 +     # note: comments, get-mark, ls-tree, and cat-blob requests may
 +     # appear anywhere in the input, except within a data command. Any
 +     # form of the data command always escapes the related input from
 +     # comment processing.
       #
       # In case it is not clear, the '#' that starts the comment
       # must be the first character on that line (an lf
       # preceded it).
       #
  
 +  get_mark ::= 'get-mark' sp idnum lf;
    cat_blob ::= 'cat-blob' sp (hexsha1 | idnum) lf;
    ls_tree  ::= 'ls' sp (hexsha1 | idnum) sp path_str lf;
  
@@@ -373,7 -372,6 +373,7 @@@ static volatile sig_atomic_t checkpoint
  static int cat_blob_fd = STDOUT_FILENO;
  
  static void parse_argv(void);
 +static void parse_get_mark(const char *p);
  static void parse_cat_blob(const char *p);
  static void parse_ls(const char *p, struct branch *b);
  
@@@ -1694,13 -1692,13 +1694,13 @@@ static int update_branch(struct branch 
        unsigned char old_sha1[20];
        struct strbuf err = STRBUF_INIT;
  
 -      if (read_ref(b->name, old_sha1))
 -              hashclr(old_sha1);
        if (is_null_sha1(b->sha1)) {
                if (b->delete)
 -                      delete_ref(b->name, old_sha1, 0);
 +                      delete_ref(b->name, NULL, 0);
                return 0;
        }
 +      if (read_ref(b->name, old_sha1))
 +              hashclr(old_sha1);
        if (!force_update && !is_null_sha1(old_sha1)) {
                struct commit *old_cmit, *new_cmit;
  
@@@ -1909,10 -1907,6 +1909,10 @@@ static int read_next_command(void
                        rc->prev->next = rc;
                        cmd_tail = rc;
                }
 +              if (skip_prefix(command_buf.buf, "get-mark ", &p)) {
 +                      parse_get_mark(p);
 +                      continue;
 +              }
                if (skip_prefix(command_buf.buf, "cat-blob ", &p)) {
                        parse_cat_blob(p);
                        continue;
@@@ -2594,14 -2588,12 +2594,12 @@@ static int parse_from(struct branch *b
  {
        const char *from;
        struct branch *s;
+       unsigned char sha1[20];
  
        if (!skip_prefix(command_buf.buf, "from ", &from))
                return 0;
  
-       if (b->branch_tree.tree) {
-               release_tree_content_recursive(b->branch_tree.tree);
-               b->branch_tree.tree = NULL;
-       }
+       hashcpy(sha1, b->branch_tree.versions[1].sha1);
  
        s = lookup_branch(from);
        if (b == s)
@@@ -2616,14 -2608,16 +2614,16 @@@
                struct object_entry *oe = find_mark(idnum);
                if (oe->type != OBJ_COMMIT)
                        die("Mark :%" PRIuMAX " not a commit", idnum);
-               hashcpy(b->sha1, oe->idx.sha1);
-               if (oe->pack_id != MAX_PACK_ID) {
-                       unsigned long size;
-                       char *buf = gfi_unpack_entry(oe, &size);
-                       parse_from_commit(b, buf, size);
-                       free(buf);
-               } else
-                       parse_from_existing(b);
+               if (hashcmp(b->sha1, oe->idx.sha1)) {
+                       hashcpy(b->sha1, oe->idx.sha1);
+                       if (oe->pack_id != MAX_PACK_ID) {
+                               unsigned long size;
+                               char *buf = gfi_unpack_entry(oe, &size);
+                               parse_from_commit(b, buf, size);
+                               free(buf);
+                       } else
+                               parse_from_existing(b);
+               }
        } else if (!get_sha1(from, b->sha1)) {
                parse_from_existing(b);
                if (is_null_sha1(b->sha1))
@@@ -2632,6 -2626,11 +2632,11 @@@
        else
                die("Invalid ref name or SHA1 expression: %s", from);
  
+       if (b->branch_tree.tree && hashcmp(sha1, b->branch_tree.versions[1].sha1)) {
+               release_tree_content_recursive(b->branch_tree.tree);
+               b->branch_tree.tree = NULL;
+       }
+ 
        read_next_command();
        return 1;
  }
@@@ -2925,23 -2924,6 +2930,23 @@@ static void cat_blob(struct object_entr
                free(buf);
  }
  
 +static void parse_get_mark(const char *p)
 +{
 +      struct object_entry *oe = oe;
 +      char output[42];
 +
 +      /* get-mark SP <object> LF */
 +      if (*p != ':')
 +              die("Not a mark: %s", p);
 +
 +      oe = find_mark(parse_mark_ref_eol(p));
 +      if (!oe)
 +              die("Unknown mark: %s", command_buf.buf);
 +
 +      snprintf(output, sizeof(output), "%s\n", sha1_to_hex(oe->idx.sha1));
 +      cat_blob_write(output, 41);
 +}
 +
  static void parse_cat_blob(const char *p)
  {
        struct object_entry *oe = oe;
@@@ -3263,8 -3245,6 +3268,8 @@@ static int parse_one_feature(const cha
                option_import_marks(arg, from_stream, 1);
        } else if (skip_prefix(feature, "export-marks=", &arg)) {
                option_export_marks(arg);
 +      } else if (!strcmp(feature, "get-mark")) {
 +              ; /* Don't die - this feature is supported */
        } else if (!strcmp(feature, "cat-blob")) {
                ; /* Don't die - this feature is supported */
        } else if (!strcmp(feature, "relative-marks")) {