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.
% hg debuginstall
Checking encoding (ascii)...
Checking extensions...
Checking templates...
Checking patch...
Checking commit editor...
Checking username...
No problems detected
% hg debuginstall with no username
Checking encoding (ascii)...
Checking extensions...
Checking templates...
Checking patch...
Checking commit editor...
Checking username...
no username supplied (see "hg help config")
(specify a username in your .hgrc file)
1 problems detected, please check your install!