commit: do not lose SQUASH_MSG contents

commit: do not lose SQUASH_MSG contents

When concluding a conflicted "git merge --squash", the command
failed to read SQUASH_MSG that was prepared by "git merge", and
showed only the "# Conflicts:" list of conflicted paths.

Place the contents from SQUASH_MSG at the beginning, just like we
show the commit log skeleton first when concluding a normal merge,
and then show the "# Conflicts:" list, to help the user write the
log message for the resulting commit.

Test by Junio C Hamano <gitster@pobox.com>.

Signed-off-by: Sven Strickroth <sven@cs-ware.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>