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

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