Add custom subject prefix support to format-patch (take 3)

Add a new option to git-format-patch, entitled --subject-prefix that allows
control of the subject prefix '[PATCH]'. Using this option, the text 'PATCH' is
replaced with whatever input is provided to the option. This allows easily
generating patches like '[PATCH 2.6.21-rc3]' or properly numbered series like
'[-mm3 PATCH N/M]'. This patch provides the implementation and documentation.

Signed-off-by: Robin H. Johnson <robbat2@gentoo.org>
Signed-off-by: Junio C Hamano <junkio@cox.net>
diff --git a/revision.h b/revision.h
index 55e6b53..5f3f628 100644
--- a/revision.h
+++ b/revision.h
@@ -78,6 +78,7 @@
 	const char	*add_signoff;
 	const char	*extra_headers;
 	const char	*log_reencode;
+	const char	*subject_prefix;
 	int		no_inline;
 
 	/* Filter by commit log message */