gitmirror: introduce gitmirror
gitmirror is just another name of knotmirror/v2. Current Knotmirror will
be gradually replaced by gitmirror except the knot-relay part.
gitmirror implements cross-repo operations like diff or merge-check
which cannot be done from individual knots when PR source is from
different Knot.
Signed-off-by: Seongmin Lee <git@boltless.me>