bash-completion: fallback to default completion

If we can't provide any completions, then fallback to the standard
bash & readline ones.  This allows completion based on the user's
settings (e.g. local paths) to kick in.

Bug: https://crbug.com/gerrit/14797
Test: `repo rebase ./src/<tab>` works in a CrOS checkout
Change-Id: Iced343c4fc6fd3a932aab99875c1346687d187b6
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/312902
Reviewed-by: Xin Li <delphij@google.com>
Tested-by: Mike Frysinger <vapier@google.com>
diff --git a/completion.bash b/completion.bash
index 0b52d29..3c1fd68 100644
--- a/completion.bash
+++ b/completion.bash
@@ -118,4 +118,6 @@
   return 0
 }
 
-complete -F __complete_repo repo
+# Fallback to the default complete methods if we aren't able to provide anything
+# useful.  This will allow e.g. local paths to be used when it makes sense.
+complete -F __complete_repo -o bashdefault -o default repo