contrib/completion: complete options that take refs for format-patch

contrib/completion: complete options that take refs for format-patch

The completion for format-patch currently suggests --base=, --interdiff=
and --range-diff= as options. However, with these `=` forms of the
options, there is no space and we'd enter the `--*` case which means we
don't call the __git_complete_revlist() at the end.

Teach _git_format_patch() to complete refs in the case of --base=,
--interdiff= and --range-diff=.

Signed-off-by: Denton Liu <liu.denton@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>