format-patch: add --reroll-count=$N option

format-patch: add --reroll-count=$N option

The --reroll-count=$N option, when given a positive integer:

 - Adds " v$N" to the subject prefix specified.  As the default
   subject prefix string is "PATCH", --reroll-count=2 makes it
   "PATCH v2".

 - Prefixes "v$N-" to the names used for output files.  The cover
   letter, whose name is usually 0000-cover-letter.patch, becomes
   v2-0000-cover-letter.patch when given --reroll-count=2.

This allows users to use the same --output-directory for multiple
iterations of the same series, without letting the output for a
newer round overwrite output files from the earlier rounds.  The
user can incorporate materials from earlier rounds to update the
newly minted iteration, and use "send-email v2-*.patch" to send out
the patches belonging to the second iteration easily.

Signed-off-by: Junio C Hamano <gitster@pobox.com>