Merge branch 'pt/am-abort-fix' into maint

Various fixes around "git am" that applies a patch to a history
that is not there yet.

* pt/am-abort-fix:
am --abort: keep unrelated commits on unborn branch
am --abort: support aborting to unborn branch
am --abort: revert changes introduced by failed 3way merge
am --skip: support skipping while on unborn branch
am -3: support 3way merge on unborn branch
am --skip: revert changes introduced by failed 3way merge

diff --cc git-am.sh
Simple merge

diff --cc t/t4151-am-abort.sh
Simple merge