Teach fmt-patch to write individual files.

Teach fmt-patch to write individual files.

When called with "--stdout", it still writes to standard output.

Notable differences to git-format-patch:

	- since fmt-patch uses the standardized logging machinery, it is
	  no longer "From nobody", but "From <commit_sha1>",

	- the empty lines before and after the "---" just before the
	  diffstat are no longer there,

	- git-format-patch outputs the commit_sha1 just before the first
	  diff, which fmt-patch does not,

	- the file names are no longer output to stdout, but to stderr
	  (since stdout is freopen()ed all the time), and

	- "git fmt-patch HEAD^" does not work as expected: it outputs
	  *all* commits reachable from HEAD^!

The last one is possibly a showstopper. At least I used to call that
command quite often...

Signed-off-by: Johannes Schindelin <Johannes.Schindelin@gmx.de>
Signed-off-by: Junio C Hamano <junkio@cox.net>