[PATCH] completion: complete tracked paths for 'git diff'

When completing arguments for 'git diff', _git_diff() delegates to
__git_complete_revlist_file(), which only completes revision
references.  This is good [*], as mixing both revs and paths in a
single list to have the user pick is simply too confusing.

If no reference matches, or if '--' is given, however, _git_diff()
leaves COMPREPLY empty. Bash then falls back to default filename
completion in $PWD. This fails when 'git -C <path>' is used because
$PWD is not the target repository.

Update _git_diff() to use __git_complete_index_file() when '--' is
present, or when revision reference completion yields no matching
candidates, so that tracked paths are offered as candidates.

[Footnote]

 * In https://lore.kernel.org/git/al%[email protected]/
   SZEDER made the same argument for "git send-email 0<TAB>".

Signed-off-by: Junio C Hamano <[email protected]>
---

 * This is one of my pet peeves that I have raised a few times on
   the list but nobody reacted.  So I did a "monkey see, monkey do"
   patch without deeply understanding what is going on in the code
   paths.  When preparing the CC: list, I pulled a few folks, some
   very recognizable, some not recognizable immediately by me, out
   of "git shortlog --since=3.years" on this file.  The contribution
   by any of them looked more expertly done by whatever I did here.

   The use case is that I have a checkout of the 'todo' branch in an
   untracked subdirectory 'Meta' in my primary source tree.  I would
   do

    $ git -C Meta status wh<TAB>

   and it completes to whats-cooking.txt just fine, 'add' also adds
   it, but 'diff' dies not work, not because I have refs that 'wh'
   completes, but because bash completion is unaware that I want
   paths completed in the other directory.

 contrib/completion/git-completion.bash |  8 +++++-
 t/t9902-completion.sh                  | 40 ++++++++++++++++++++++++++
 2 files changed, 47 insertions(+), 1 deletion(-)

diff --git a/contrib/completion/git-completion.bash b/contrib/completion/git-completion.bash
index e875787710..8f5773292b 100644
--- a/contrib/completion/git-completion.bash
+++ b/contrib/completion/git-completion.bash
@@ -1947,7 +1947,10 @@ __git_diff_difftool_options="--cached --staged
 
 _git_diff ()
 {
-	__git_has_doubledash && return
+	if __git_has_doubledash; then
+		__git_complete_index_file
+		return
+	fi
 
 	case "$cur" in
 	--diff-algorithm=*)
@@ -1976,6 +1979,9 @@ _git_diff ()
 		;;
 	esac
 	__git_complete_revlist_file
+	if [ ${#COMPREPLY[@]} -eq 0 ]; then
+		__git_complete_index_file
+	fi
 }
 
 __git_mergetools_common="diffuse diffmerge ecmerge emerge kdiff3 meld opendiff
diff --git a/t/t9902-completion.sh b/t/t9902-completion.sh
index 55dc9eabfc..eecd53f097 100755
--- a/t/t9902-completion.sh
+++ b/t/t9902-completion.sh
@@ -2663,6 +2663,7 @@ test_expect_success 'setup for integration tests' '
 	echo content >file1 &&
 	echo more >file2 &&
 	git add file1 file2 &&
+	echo untracked >file3 &&
 	git commit -m one &&
 	git branch mybranch &&
 	git tag mytag
@@ -2712,6 +2713,45 @@ test_expect_success 'git -C <path> checkout uses the right repo' '
 	EOF
 '
 
+test_expect_success 'git diff completes tracked paths when no refs match' '
+	# file1 and file2 are tracked but file3 is not
+	test_completion "git diff f" <<-\EOF
+	file1
+	file2
+	EOF
+'
+
+test_expect_success 'git diff -- completes tracked paths' '
+	# file1 and file2 are tracked but file3 is not
+	test_completion "git diff -- f" <<-\EOF
+	file1
+	file2
+	EOF
+'
+
+test_expect_success 'git -C <path> diff completes tracked paths in specified repo' '
+	test_when_finished "rm -rf repo-for-diff" &&
+	git init repo-for-diff &&
+	echo content >repo-for-diff/otherfile &&
+	git -C repo-for-diff add otherfile &&
+	echo untracked >repo-for-diff/oops &&
+	git -C repo-for-diff commit -m otherfile &&
+	test_completion "git -C repo-for-diff diff o" <<-\EOF
+	otherfile
+	EOF
+'
+
+test_expect_success 'git -C <path> diff -- completes pathspecs in specified repo' '
+	test_when_finished "rm -rf repo-for-diff" &&
+	git init repo-for-diff &&
+	echo content >repo-for-diff/otherfile &&
+	git -C repo-for-diff add otherfile &&
+	git -C repo-for-diff commit -m otherfile &&
+	test_completion "git -C repo-for-diff diff -- o" <<-\EOF
+	otherfile
+	EOF
+'
+
 test_expect_success 'show completes all refs' '
 	test_completion "git show m" <<-\EOF
 	main Z
-- 
2.55.0-607-g47e9082d35