mq: preserve --git flag when folding patches
Without this, folding a git patch into a regular one downgrades the resulting
patch to a regular patch.
0 files updated, 0 files merged, 1 files removed, 0 files unresolved
created new head
1 files updated, 0 files merged, 0 files removed, 0 files unresolved
(branch merge, don't forget to commit)