completion: 'git diff' completes untracked paths as a last resort

We taught 'git diff' to first try to complete revisions (unless '--'
is present on the command line) and, failing that, to complete
tracked paths.  If this yields nothing, it lets the Bash default,
which offers paths in $PWD, kick in.

Teach it to complete untracked paths before giving up and letting
the Bash default kick in.  With this change,

    $ git -C another-directory diff un<TAB>

finds the 'untracked' file in another-directory and offers it as a
completion candidate.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

contrib/completion/git-completion.bash

@@ -1985,6 +1985,10 @@ _git_diff ()
 	if [ ${#COMPREPLY[@]} -eq 0 ]; then
 		__git_complete_index_file
 	fi
+
+	if [ ${#COMPREPLY[@]} -eq 0 ]; then
+		__git_complete_index_file "--others --directory"
+	fi
 }
 
 __git_mergetools_common="diffuse diffmerge ecmerge emerge kdiff3 meld opendiff

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 >ufile &&
 	git commit -m one &&
 	git branch mybranch &&
 	git tag mytag
@@ -2728,6 +2729,15 @@ test_expect_success 'git diff -- completes tracked paths' '
 	EOF
 '
 
+test_expect_success 'git diff [--] completes untracked paths, too' '
+	test_completion "git diff u" <<-\EOF &&
+	ufile
+	EOF
+	test_completion "git diff -- u" <<-\EOF
+	ufile
+	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 &&
@@ -2744,11 +2754,21 @@ 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 &&
+	echo untracked >repo-for-diff/untracked &&
 	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
+	test_completion "git -C repo-for-diff diff o" <<-\EOF &&
 	otherfile
 	EOF
+	test_completion "git -C repo-for-diff diff -- o" <<-\EOF &&
+	otherfile
+	EOF
+	test_completion "git -C repo-for-diff diff u" <<-\EOF &&
+	untracked
+	EOF
+	test_completion "git -C repo-for-diff diff -- u" <<-\EOF
+	untracked
+	EOF
 '
 
 test_expect_success 'show completes all refs' '