test: completion: add tests for __git_complete

Even though the function was marked as not public, it's already used in
the wild.

We should at least test basic functionality.

Signed-off-by: Felipe Contreras <felipe.contreras@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t9902-completion.sh

@@ -2380,4 +2380,12 @@ test_expect_success 'sourcing the completion script clears cached --options' '
 	verbose test -z "$__gitcomp_builtin_notes_edit"
 '
 
+test_expect_success '__git_complete' '
+	unset -f __git_wrap__git_main &&
+	__git_complete foo __git_main &&
+	__git_have_func __git_wrap__git_main &&
+	__git_complete gf _git_fetch &&
+	__git_have_func __git_wrap_git_fetch
+'
+
 test_done