format-patch: construct patch filename in one function

reopen_stdout() usually takes the oneline subject of a commit,
appends the patch suffix, prepends the output directory (if any) and
then reopens stdout as the resulting file. Now the patch filename (the
oneline subject and the patch suffix) is created in
get_patch_filename() and passed to reopen_stdout() which prepends the
output directory and reopens stdout as that file.

The original function to get the oneline description,
get_oneline_for_filename(), has been renamed to get_patch_filename() to
reflect its new functionality.

Signed-off-by: Stephen Boyd <bebarino@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>