format-patch: make zero-length subject prefixes prettier

format-patch: make zero-length subject prefixes prettier

If you give a zero-length subject prefix to format-patch
(e.g., "format-patch --subject-prefix="), we will print the
ugly:

  Subject: [ 1/2] your subject here

because we always insert a space between the prefix and
numbering. Requiring the user to provide the space in their
prefix would be more flexible, but would break existing
usage. This patch provides a DWIM and suppresses the space
for zero-length prefixes, under the assumption that nobody
actually wants "[ 1/2]".

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>