format-patch: "--rfc=-(WIP)" appends to produce [PATCH (WIP)]
In the previous step, the "--rfc" option of "format-patch" learned
to take an optional string value to prepend to the subject prefix,
so that --rfc=WIP can give "[WIP PATCH]".
There may be cases in which the extra string wants to come after the
subject prefix. Extend the mechanism to allow "--rfc=-(WIP)" [*] to
signal that the extra string is to be appended instead of getting
prepended, resulting in "[PATCH (WIP)]".
In the documentation, discourage (ab)using "--rfc=-RFC" to say
"[PATCH RFC]" just to be different, when "[RFC PATCH]" is the norm.
[Footnote]
* The syntax takes inspiration from Perl's open syntax that opens
pipes "open fh, '|-', 'cmd'", where the dash signals "the other
stuff comes here".
Signed-off-by: Junio C Hamano <gitster@pobox.com>