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>

git-rebase--interactive.sh

@@ -349,11 +349,9 @@ nth_string () {
 
 make_squash_message () {
 	if test -f "$SQUASH_MSG"; then
-		# We want to be careful about matching only the commit
-		# message comment lines generated by this function.
-		# "[snrt][tdh]" matches the nth_string endings.
-		COUNT=$(($(sed -n "s/^# Th[^0-9]*\([1-9][0-9]*\)[snrt][tdh] commit message.*:/\1/p" \
-			< "$SQUASH_MSG" | sed -ne '$p')+1))
+		COUNT=$(($(sed -n \
+			-e "1s/^# This is a combination of \(.*\) commits\./\1/p" \
+			-e "q" < "$SQUASH_MSG")+1))
 		echo "# This is a combination of $COUNT commits."
 		sed -e 1d -e '2,/^./{
 			/^$/d
@@ -376,9 +374,6 @@ make_squash_message () {
 		echo
 		echo "# The $(nth_string $COUNT) commit message will be skipped:"
 		echo
-		# Comment the lines of the commit message out using
-		# "#	" rather than "# " to make them less likely to
-		# confuse the sed regexp above.
 		git cat-file commit $2 | sed -e '1,/^$/d' -e 's/^/#	/'
 		;;
 	esac