Re: [PATCH 2/2] difftool --dir-diff: symlink all files matching the working tree

Some users like to edit files in their diff tool when using "git
difftool --dir-diff --symlink" to compare against the working tree but
difftool currently only created symlinks when a file contains unstaged
changes.

Change this behaviour so that symlinks are created whenever the
right-hand side of the comparison has the same SHA1 as the file in the
working tree.

Note that textconv filters are handled in the same way as by git-diff
and if a clean filter is not the inverse of its smudge filter we already
get a null SHA1 from "diff --raw" and will symlink the file without
going through the new hash-object based check.

Reported-by: Matt McClure <redacted>
Signed-off-by: John Keeping <redacted>
---
 Documentation/git-difftool.txt |  4 +++-
 git-difftool.perl              | 21 ++++++++++++++++++---
 t/t7800-difftool.sh            | 14 ++++++++++++++
 3 files changed, 35 insertions(+), 4 deletions(-)

diff --git a/Documentation/git-difftool.txt b/Documentation/git-difftool.txt
index e575fea..8361e6e 100644
--- a/Documentation/git-difftool.txt
+++ b/Documentation/git-difftool.txt

@@ -72,7 +72,9 @@ with custom merge tool commands and has the same value as `$MERGED`.
 --symlinks::
 --no-symlinks::
        'git difftool''s default behavior is create symlinks to the
-       working tree when run in `--dir-diff` mode.
+       working tree when run in `--dir-diff` mode and the right-hand
+       side of the comparison yields the same content as the file in
+       the working tree.
 +
 Specifying `--no-symlinks` instructs 'git difftool' to create copies
 instead.  `--no-symlinks` is the default on Windows.

diff --git a/git-difftool.perl b/git-difftool.perl
index 0a90de4..5f093ae 100755
--- a/git-difftool.perl
+++ b/git-difftool.perl

@@ -83,6 +83,21 @@ sub exit_cleanup
        exit($status | ($status >> 8));
 }

+sub use_wt_file
+{
+       my ($repo, $workdir, $file, $sha1, $symlinks) = @_;
+       my $null_sha1 = '0' x 40;
+
+       if ($sha1 eq $null_sha1) {
+               return 1;
+       } elsif (not $symlinks) {
+               return 0;
+       }
+
+       my $wt_sha1 = $repo->command_oneline('hash-object', "$workdir/$file");
+       return $sha1 eq $wt_sha1;
+}
+
 sub setup_dir_diff
 {
        my ($repo, $workdir, $symlinks) = @_;

@@ -159,10 +174,10 @@ EOF
                }

                if ($rmode ne $null_mode) {
-                       if ($rsha1 ne $null_sha1) {
-                               $rindex .= "$rmode $rsha1\t$dst_path\0";
-                       } else {
+                       if (use_wt_file($repo, $workdir, $dst_path, $rsha1, $symlinks)) {
                                push(@working_tree, $dst_path);
+                       } else {
+                               $rindex .= "$rmode $rsha1\t$dst_path\0";
                        }
                }
        }

diff --git a/t/t7800-difftool.sh b/t/t7800-difftool.sh
index eb1d3f8..8102ce1 100755
--- a/t/t7800-difftool.sh
+++ b/t/t7800-difftool.sh

@@ -370,6 +370,20 @@ test_expect_success PERL 'difftool --dir-diff' '
        echo "$diff" | stdin_contains file
 '

+write_script .git/CHECK_SYMLINKS <<\EOF &&