Merge branch 'jk/reset-ident-time-per-commit' into maint

Not-so-recent rewrite of "git am" that started making internal calls into the commit machinery had an unintended regression, in that no matter how many seconds it took to apply many patches, the resulting committer timestamp for the resulting commits were all the same. * jk/reset-ident-time-per-commit: am: reset cached ident date for each patch