Teach bash how to complete git-format-patch.

Teach bash how to complete git-format-patch.

Provide completion for currently known long options supported by
git-format-patch as well as the revision list specification argument,
which is generally either a refname or in the form a..b.

Since _git_log was the only code that knew how to complete a..b, but
we want to start adding option support to _git_log also refactor the
a..b completion logic out into its own function.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>
Signed-off-by: Junio C Hamano <junkio@cox.net>