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

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>