Merge branch 'tr/merge-edit-guidance'

* tr/merge-edit-guidance:
merge: add instructions to the commit message when editing

diff --combined builtin/merge.c
index bfb75476aab154e430d6247073e0bbcd83c992f6,41d835e3f7341410bd0cdb8d6ff730c945ea2457..62c7b681778d6a3fcffc792cc8a131cbd7f45f90
--- 1/builtin/merge.c
--- 2/builtin/merge.c
+++ b/builtin/merge.c
@@@ -48,7 -48,7 +48,7 @@@ static const char * const builtin_merge
  
  static int show_diffstat = 1, shortlog_len = -1, squash;
  static int option_commit = 1, allow_fast_forward = 1;
 -static int fast_forward_only, option_edit;
 +static int fast_forward_only, option_edit = -1;
  static int allow_trivial = 1, have_message;
  static int overwrite_ignore = 1;
  static struct strbuf merge_msg = STRBUF_INIT;
@@@ -193,7 -193,7 +193,7 @@@ static struct option builtin_merge_opti
                "create a single commit instead of doing a merge"),
        OPT_BOOLEAN(0, "commit", &option_commit,
                "perform a commit if the merge succeeds (default)"),
 -      OPT_BOOLEAN('e', "edit", &option_edit,
 +      OPT_BOOL('e', "edit", &option_edit,
                "edit message before committing"),
        OPT_BOOLEAN(0, "ff", &allow_fast_forward,
                "allow fast-forward (default)"),
@@@ -885,20 -885,30 +885,30 @@@ static void abort_commit(const char *er
        exit(1);
  }
  
+ static const char merge_editor_comment[] =
+ N_("Please enter a commit message to explain why this merge is necessary,\n"
+    "especially if it merges an updated upstream into a topic branch.\n"
+    "\n"
+    "Lines starting with '#' will be ignored, and an empty message aborts\n"
+    "the commit.\n");
+ 
  static void prepare_to_commit(void)
  {
        struct strbuf msg = STRBUF_INIT;
+       const char *comment = _(merge_editor_comment);
        strbuf_addbuf(&msg, &merge_msg);
        strbuf_addch(&msg, '\n');
+       if (0 < option_edit)
+               strbuf_add_lines(&msg, "# ", comment, strlen(comment));
        write_merge_msg(&msg);
        run_hook(get_index_file(), "prepare-commit-msg",
                 git_path("MERGE_MSG"), "merge", NULL, NULL);
 -      if (option_edit) {
 +      if (0 < option_edit) {
                if (launch_editor(git_path("MERGE_MSG"), NULL, NULL))
                        abort_commit(NULL);
        }
        read_merge_msg(&msg);
 -      stripspace(&msg, option_edit);
 +      stripspace(&msg, 0 < option_edit);
        if (!msg.len)
                abort_commit(_("Empty commit message."));
        strbuf_release(&merge_msg);
@@@ -1099,33 -1109,6 +1109,33 @@@ static void write_merge_state(void
        close(fd);
  }
  
 +static int default_edit_option(void)
 +{
 +      static const char name[] = "GIT_MERGE_AUTOEDIT";
 +      const char *e = getenv(name);
 +      struct stat st_stdin, st_stdout;
 +
 +      if (have_message)
 +              /* an explicit -m msg without --[no-]edit */
 +              return 0;
 +
 +      if (e) {
 +              int v = git_config_maybe_bool(name, e);
 +              if (v < 0)
 +                      die("Bad value '%s' in environment '%s'", e, name);
 +              return v;
 +      }
 +
 +      /* Use editor if stdin and stdout are the same and is a tty */
 +      return (!fstat(0, &st_stdin) &&
 +              !fstat(1, &st_stdout) &&
 +              isatty(0) &&
 +              st_stdin.st_dev == st_stdout.st_dev &&
 +              st_stdin.st_ino == st_stdout.st_ino &&
 +              st_stdin.st_mode == st_stdout.st_mode);
 +}
 +
 +
  int cmd_merge(int argc, const char **argv, const char *prefix)
  {
        unsigned char result_tree[20];
@@@ -1318,9 -1301,6 +1328,9 @@@
                }
        }
  
 +      if (option_edit < 0)
 +              option_edit = default_edit_option();
 +
        if (!use_strategies) {
                if (!remoteheads->next)
                        add_strategies(pull_twohead, DEFAULT_TWOHEAD);