mailinfo: Remove only one set of square brackets

mailinfo: Remove only one set of square brackets

git-format-patch prepends patches with a [PATCH x/n] prefix, but
mailinfo used to remove any number of square-bracket pairs and
the content between them. This prevents one from using a commit
subject like this:

  [ and ] must be allowed as input

Removing the square bracket pair from this rather clumsily
constructed subject line loses important information, so we must
take care not to.

This patch causes the subject stripping to stop after it has
encountered one pair of square brackets.

One possible downside of this patch is that the patch-handling
programs will now fail at removing author-added square-brackets
to be removed, such as

  [RFC][PATCH x/n]

However, since format-patch only adds one set of square brackets,
this behaviour is quite easily undesrstood and defended while the
previous behaviour is not.

Signed-off-by: Andreas Ericsson <ae@op5.se>
Signed-off-by: Junio C Hamano <gitster@pobox.com>