format-patch: refactor get_patch_filename

The get_patch_filename function expects a commit argument
and uses it to get the sanitized subject line when making a
patch filename. However, we also want to use this same
function for the cover letter, which does not have a commit
object. The current solution is to create a fake commit with
the subject "cover letter". Instead, let's make the
get_patch_filename interface more flexibile, and allow
passing a direct subject.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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