mailinfo: -b option keeps [bracketed] strings that is not a [PATCH] marker

mailinfo: -b option keeps [bracketed] strings that is not a [PATCH] marker

By default, we remove leading [bracketed] [strings] from the Subject:
header when coming up with the summary of the patch.  This is because
there are mailing lists etc that add their own headers to the subject, and
they know they can add things in brackets. The most obvious example is the
Linux kernel security list.  Their emails look like

 	Subject: [Security] [patch] random: make get_random_int() more random

and other people mangle Subject: themselves in a similar way, e.g.:

 	Subject: [PATCH -rc] [BUGFIX] x86: fix kernel_trap_sp()
 	Subject: [BUGFIX][PATCH] fix bad page removal from LRU (Was Re: [RFC][PATCH] ..

even though "fix" is more than enough cue to mark it as a [BUGFIX].

Some projects however want to keep these bracketed strings.  With this
option, we remove only [bracketed strings that contain word PATCH], so we
will turn things like these

	[PATCH] [mailinfo] -b ...
	[PATCH v2] [mailinfo] -b ...
	[PATCH (v2) 1/4] [mailinfo] -b ...

into

	[mailinfo] -b ...

This lacks tests and integration to the "git am" toolchain to be useful,
but it is a start.

Signed-off-by: Junio C Hamano <gitster@pobox.com>