Merge branch 'jk/diff-do-not-reuse-wtf-needs-cleaning' into maint

There is an optimization used in "git diff $treeA $treeB" to borrow
an already checked-out copy in the working tree when it is known to
be the same as the blob being compared, expecting that open/mmap of
such a file is faster than reading it from the object store, which
involves inflating and applying delta.  This however kicked in even
when the checked-out copy needs to go through the convert-to-git
conversion (including the clean filter), which defeats the whole
point of the optimization.  The optimization has been disabled when
the conversion is necessary.

* jk/diff-do-not-reuse-wtf-needs-cleaning:
diff: do not reuse worktree files that need "clean" conversion

diff --combined diff.c
index fa78fc189cd7225767817a2d24ef6c3b19bef9d8,918cedc4ad3f4ef21f5b8d04325bd7286ef83de7..abff2eec2b0604786b09ff0ac7fae24e1df6677f
--- 1/diff.c
--- 2/diff.c
+++ b/diff.c
@@@ -26,7 -26,6 +26,7 @@@
  #endif
  
  static int diff_detect_rename_default;
 +static int diff_compaction_heuristic; /* experimental */
  static int diff_rename_limit_default = 400;
  static int diff_suppress_blank_empty;
  static int diff_use_color_default = -1;
@@@ -169,11 -168,6 +169,11 @@@ long parse_algorithm_value(const char *
   * never be affected by the setting of diff.renames
   * the user happens to have in the configuration file.
   */
 +void init_diff_ui_defaults(void)
 +{
 +      diff_detect_rename_default = 1;
 +}
 +
  int git_diff_ui_config(const char *var, const char *value, void *cb)
  {
        if (!strcmp(var, "diff.color") || !strcmp(var, "color.diff")) {
@@@ -190,10 -184,6 +190,10 @@@
                diff_detect_rename_default = git_config_rename(var, value);
                return 0;
        }
 +      if (!strcmp(var, "diff.compactionheuristic")) {
 +              diff_compaction_heuristic = git_config_bool(var, value);
 +              return 0;
 +      }
        if (!strcmp(var, "diff.autorefreshindex")) {
                diff_auto_refresh_index = git_config_bool(var, value);
                return 0;
@@@ -2682,6 -2672,13 +2682,13 @@@ static int reuse_worktree_file(const ch
        if (!FAST_WORKING_DIRECTORY && !want_file && has_sha1_pack(sha1))
                return 0;
  
+       /*
+        * Similarly, if we'd have to convert the file contents anyway, that
+        * makes the optimization not worthwhile.
+        */
+       if (!want_file && would_convert_to_git(name))
+               return 0;
+ 
        len = strlen(name);
        pos = cache_name_pos(name, len);
        if (pos < 0)
@@@ -3283,8 -3280,6 +3290,8 @@@ void diff_setup(struct diff_options *op
        options->use_color = diff_use_color_default;
        options->detect_rename = diff_detect_rename_default;
        options->xdl_opts |= diff_algorithm;
 +      if (diff_compaction_heuristic)
 +              DIFF_XDL_SET(options, COMPACTION_HEURISTIC);
  
        options->orderfile = diff_order_file_cfg;
  
@@@ -3805,10 -3800,6 +3812,10 @@@ int diff_opt_parse(struct diff_options 
                DIFF_XDL_SET(options, IGNORE_WHITESPACE_AT_EOL);
        else if (!strcmp(arg, "--ignore-blank-lines"))
                DIFF_XDL_SET(options, IGNORE_BLANK_LINES);
 +      else if (!strcmp(arg, "--compaction-heuristic"))
 +              DIFF_XDL_SET(options, COMPACTION_HEURISTIC);
 +      else if (!strcmp(arg, "--no-compaction-heuristic"))
 +              DIFF_XDL_CLR(options, COMPACTION_HEURISTIC);
        else if (!strcmp(arg, "--patience"))
                options->xdl_opts = DIFF_WITH_ALG(options, PATIENCE_DIFF);
        else if (!strcmp(arg, "--histogram"))