rebase -i: subsume die_failed_squash under die_with_patch

die_failed_squash is the error sequence for when a squash or fixup
command fails. Detect in die_with_patch whether we are in the middle of
a squash or fixup and insert squash_msg as commit message if that is the
case (copy the die_failed_squash code). Call die_with_patch instead of
die_failed_squash from the squash/fixup implementation so that the pick
error code looks the same both for ordinary picks and for
squashes/fixups. Remove die_failed_squash since it is no longer used.

Complete the patch set.