format-patch: "--rfc=-(WIP)" appends to produce [PATCH (WIP)]

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>