mailinfo: with -b, keep space after [foo]

mailinfo: with -b, keep space after [foo]

The logic for the -b mode, where [PATCH] is dropped but [foo] is not,
silently ate all spaces after the ].

Fix this by keeping the next isspace() character, if there is any.
Being more thorough is pointless, as the later cleanup_space() call
will normalize any sequence of whitespace to a single ' '.

Signed-off-by: Thomas Rast <trast@student.ethz.ch>
Signed-off-by: Junio C Hamano <gitster@pobox.com>