pretty.c: add %(count) and %(total) placeholders

pretty.c: add %(count) and %(total) placeholders

In many commands we can customize the output through the "--format" or
the "--pretty" options. This patch adds two new placeholders used mainly
when there's a range of commits that we want to show.

Currently these two placeholders are not usable as they're coupled with
the rev_info->nr and rev_info->total fields, fields that are used only
by the format-patch numbered email subjects.

Teach repo_format_commit_message() the %(count) and %(total)
placeholders.

Signed-off-by: Mirko Faina <mroik@delayed.space>
Signed-off-by: Junio C Hamano <gitster@pobox.com>