format-patch: show 0/1 and 1/1 for singleton patch with cover letter

format-patch: show 0/1 and 1/1 for singleton patch with cover letter

Change the default behavior of git-format-patch to generate numbered
sequence of 0/1 and 1/1 when generating both a cover-letter and a single
patch. This standardizes the cover letter to have 0/N which helps
distinguish the cover letter from the patch itself. Since the behavior
is easily changed via configuration as well as the use of -n and -N this
should be acceptable default behavior.

Add tests for the new default behavior.

Signed-off-by: Jacob Keller <jacob.keller@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>