[RFC] series headers

I'd like to be able to get format-patch to produce a [PATCH 0/N] message, with a message that's actually in my repository, plus various goodies generated either from diffing the ends of the series or by running through the log an extra time to pick up summary information.

The second best idea I have for this currently is to have a commit at the end of the series which specifies which has the same tree as its parent and has a message with a line "Since <sha1 of the first commit of the series>" and has the text. This goes at the end of the series, because it describes the state with all of the changes made, which is only a good description of a commit at the end of the series, not a commit at the start of the series. Making it [PATCH 0/N] is just because it belongs first in presentation, regardless of whether the other commits are presented with recent commits first or last.

The better idea I just had was to have format-patch notice if the "until" side is a tag object instead of a commit, and generate a [0/N] with the tag message.

As far as implementing this... would it be sane to make struct rev_info.commit_format a callback, so that the code to generate an email message can be somewhere that's easy to use to generate an email that isn't for a commit in the log? I don't *think* git's quite fast enough for the indirect jump to a callback instead of an if tree for an enum will actually hurt us.

	-Daniel
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