archive: specfile support (--pretty=format: in archive files)

archive: specfile support (--pretty=format: in archive files)

Add support for a new attribute, specfile.  Files marked as being
specfiles are expanded by git-archive when they are written to an
archive.  It has no effect on worktree files.  The same placeholders
as those for the option --pretty=format: of git-log et al. can be
used.

The attribute is useful for creating auto-updating specfiles.  It is
limited by the underlying function format_commit_message(), though.
E.g. currently there is no placeholder for git-describe like output,
and expanded specfiles can't contain NUL bytes.  That can be fixed
in format_commit_message() later and will then benefit users of
git-log, too.

Signed-off-by: Rene Scharfe <rene.scharfe@lsrfire.ath.cx>
Signed-off-by: Junio C Hamano <gitster@pobox.com>