Merge branch 'mh/fast-import-optimize-current-from' into maint

Often a fast-import stream builds a new commit on top of the previous commit it built, and it often unconditionally emits a "from" command to specify the first parent, which can be omitted in such a case. This caused fast-import to forget the tree of the previous commit and then re-read it from scratch, which was inefficient. Optimize for this common case. * mh/fast-import-optimize-current-from: fast-import: do less work when given "from" matches current branch head