Merge branch 'ss/commit-squash-msg' into maint

When "git merge --squash" stopped due to conflict, the concluding
"git commit" failed to read in the SQUASH_MSG that shows the log
messages from all the squashed commits.

* ss/commit-squash-msg:
  commit: do not lose SQUASH_MSG contents

diff --cc builtin/commit.c
Simple merge

diff --cc t/t7600-merge.sh
Simple merge