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