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

diff --git a/contrib/completion/git-completion.bash b/contrib/completion/git-completion.bash
index 76181e8..d35b4f3 100644
--- a/contrib/completion/git-completion.bash
+++ b/contrib/completion/git-completion.bash
@@ -1985,6 +1985,10 @@
 	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

diff --git a/t/t9902-completion.sh b/t/t9902-completion.sh
index 32e5d48..b889ec8 100755
--- a/t/t9902-completion.sh
+++ b/t/t9902-completion.sh
@@ -2664,6 +2664,7 @@
 	echo more >file2 &&
 	git add file1 file2 &&
 	echo untracked >file3 &&
+	echo untracked >ufile &&
 	git commit -m one &&
 	git branch mybranch &&
 	git tag mytag
@@ -2726,6 +2727,16 @@
 	EOF
 '
 
+test_expect_success 'git diff [--] completes untracked paths, too' '
+	# ufile is not tracked and there is no ref that begins with u
+	test_completion "git diff u" <<-\EOF &&
+	ufile
+	EOF
+	test_completion "git diff -- u" <<-\EOF
+	ufile
+	EOF
+'
+
 test_expect_success 'git -C <path> diff completes paths in specified repo' '
 	test_when_finished "rm -rf repo-for-diff" &&
 	git init repo-for-diff &&
@@ -2735,6 +2746,7 @@
 	git -C repo-for-diff add lostfile &&
 	git -C repo-for-diff commit -m otherfile &&
 	echo untracked >repo-for-diff/oops &&
+	echo untracked >repo-for-diff/ufile &&
 	rm -f repo-for-diff/lostfile &&
 
 	test_completion "git -C repo-for-diff diff o" <<-\EOF &&
@@ -2743,13 +2755,19 @@
 	test_completion "git -C repo-for-diff diff l" <<-\EOF &&
 	lostfile
 	EOF
+	test_completion "git -C repo-for-diff diff u" <<-\EOF &&
+	ufile
+	EOF
 
 	test_completion "git -C repo-for-diff diff -- o" <<-\EOF &&
 	otherfile
 	EOF
-	test_completion "git -C repo-for-diff diff -- l" <<-\EOF
+	test_completion "git -C repo-for-diff diff -- l" <<-\EOF &&
 	lostfile
 	EOF
+	test_completion "git -C repo-for-diff diff -- u" <<-\EOF
+	ufile
+	EOF
 '
 
 test_expect_success 'show completes all refs' '