Teach difftool's --dir-diff mode to use symlinks to represent
files from the working copy, and make it the default behavior
for the non-Windows platforms.

Using symlinks is simpler and safer since we do not need to
worry about copying files back into the worktree.
The old behavior is still available as --no-symlinks.

Signed-off-by: David Aguilar <dav...@gmail.com>
---
Handles the case where an editor unlinks the original symlink,
replacing it with a file.

 Documentation/git-difftool.txt |    8 ++++++++
 git-difftool.perl              |   33 +++++++++++++++++++++++----------
 2 files changed, 31 insertions(+), 10 deletions(-)

diff --git a/Documentation/git-difftool.txt b/Documentation/git-difftool.txt
index 31fc2e3..313d54e 100644
--- a/Documentation/git-difftool.txt
+++ b/Documentation/git-difftool.txt
@@ -66,6 +66,14 @@ of the diff post-image.  `$MERGED` is the name of the file 
which is
 being compared. `$BASE` is provided for compatibility
 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.
++
+       Specifying `--no-symlinks` instructs 'git difftool' to create
+       copies instead.  `--no-symlinks` is the default on Windows.
+
 --tool-help::
        Print a list of diff tools that may be used with `--tool`.
 
diff --git a/git-difftool.perl b/git-difftool.perl
index 2ae344c..a5b371f 100755
--- a/git-difftool.perl
+++ b/git-difftool.perl
@@ -92,7 +92,7 @@ sub print_tool_help
 
 sub setup_dir_diff
 {
-       my ($repo, $workdir) = @_;
+       my ($repo, $workdir, $symlinks) = @_;
 
        # Run the diff; exit immediately if no diff found
        # 'Repository' and 'WorkingCopy' must be explicitly set to insure that
@@ -209,8 +209,13 @@ sub setup_dir_diff
                unless (-d "$rdir/$dir") {
                        mkpath("$rdir/$dir") or die $!;
                }
-               copy("$workdir/$file", "$rdir/$file") or die $!;
-               chmod(stat("$workdir/$file")->mode, "$rdir/$file") or die $!;
+               if ($symlinks) {
+                       symlink("$workdir/$file", "$rdir/$file") or die $!;
+               } else {
+                       copy("$workdir/$file", "$rdir/$file") or die $!;
+                       my $mode = stat("$workdir/$file")->mode;
+                       chmod($mode, "$rdir/$file") or die $!;
+               }
        }
 
        # Changes to submodules require special treatment. This loop writes a
@@ -271,6 +276,7 @@ sub main
                gui => undef,
                help => undef,
                prompt => undef,
+               symlinks => $^O ne 'MSWin32' && $^O ne 'msys',
                tool_help => undef,
        );
        GetOptions('g|gui!' => \$opts{gui},
@@ -278,6 +284,8 @@ sub main
                'h' => \$opts{help},
                'prompt!' => \$opts{prompt},
                'y' => sub { $opts{prompt} = 0; },
+               'symlinks' => \$opts{symlinks},
+               'no-symlinks' => sub { $opts{symlinks} = 0; },
                't|tool:s' => \$opts{difftool_cmd},
                'tool-help' => \$opts{tool_help},
                'x|extcmd:s' => \$opts{extcmd});
@@ -316,7 +324,7 @@ sub main
        # will invoke a separate instance of 'git-difftool--helper' for
        # each file that changed.
        if (defined($opts{dirdiff})) {
-               dir_diff($opts{extcmd});
+               dir_diff($opts{extcmd}, $opts{symlinks});
        } else {
                file_diff($opts{prompt});
        }
@@ -324,13 +332,13 @@ sub main
 
 sub dir_diff
 {
-       my ($extcmd) = @_;
+       my ($extcmd, $symlinks) = @_;
 
        my $rc;
        my $repo = Git->repository();
 
        my $workdir = find_worktree($repo);
-       my ($a, $b, @working_tree) = setup_dir_diff($repo, $workdir);
+       my ($a, $b, @worktree) = setup_dir_diff($repo, $workdir, $symlinks);
        if (defined($extcmd)) {
                $rc = system($extcmd, $a, $b);
        } else {
@@ -342,13 +350,18 @@ sub dir_diff
 
        # If the diff including working copy files and those
        # files were modified during the diff, then the changes
-       # should be copied back to the working tree
-       for my $file (@working_tree) {
-               if (-e "$b/$file" && compare("$b/$file", "$workdir/$file")) {
+       # should be copied back to the working tree.
+       # Do not copy back files when symlinks are used and the
+       # external tool did not replace the original link with a file.
+       for my $file (@worktree) {
+               next if $symlinks && -l "$b/$file";
+               if (-f "$b/$file" && compare("$b/$file", "$workdir/$file")) {
                        copy("$b/$file", "$workdir/$file") or die $!;
-                       chmod(stat("$b/$file")->mode, "$workdir/$file") or die 
$!;
+                       my $mode = stat("$b/$file")->mode;
+                       chmod($mode, "$workdir/$file") or die $!;
                }
        }
+       exit(0);
 }
 
 sub file_diff
-- 
1.7.7.2.448.gee6df

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