completion: silence "fatal: Not a git repository" error

completion: silence "fatal: Not a git repository" error

It is possible that a user is trying to run a git command and fail
to realize that they are not in a git repository or working tree.
When trying to complete an operation, __git_refs would fall to a
degenerate case and attempt to use "git for-each-ref", which would
emit the error.

Hide this error message coming from "git for-each-ref".

Signed-off-by: John Szakmeister <redacted>
Signed-off-by: Junio C Hamano <redacted>