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.
#!/bin/sh
hg init dir
cd dir
echo bleh > bar
hg add bar
hg ci -m 'add bar'
hg cp bar foo
echo >> bar
hg ci -m 'cp bar foo; change bar'
hg debugrename foo
hg debugindex .hg/store/data/bar.i