rebase -i: Simplify commit counting for generated commit messages

rebase -i: Simplify commit counting for generated commit messages

Read the old count from the first line of the old commit message
rather than counting the number of commit message blocks in the file.
This is simpler, faster, and more robust (e.g., it cannot be confused
by strange commit message contents).

Signed-off-by: Michael Haggerty <mhagger@alum.mit.edu>
Signed-off-by: Junio C Hamano <gitster@pobox.com>