Merge branch 'pg/maint-1.7.9-am-where-is-patch' into maint

When "git am" failed, old timers knew to check .git/rebase-apply/patch
to see what went wrong, but we never told the users about it.

* pg/maint-1.7.9-am-where-is-patch:
am: indicate where a failed patch is to be found