format-patch: format.from gives the default for --from

This helps users who would prefer format-patch to default to --from,
and makes it easier to change the default in the future.

Signed-off-by: Josh Triplett <josh@joshtriplett.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

contrib/completion/git-completion.bash

@@ -2085,6 +2085,7 @@ _git_config ()
 		format.attach
 		format.cc
 		format.coverLetter
+		format.from
 		format.headers
 		format.numbered
 		format.pretty