rebase: keep original mq patch format (Issue1574)
Rebase now doesn't make assumptions about which format has been
used for a mq patch (git or normal).
Before finalizing a patch it keeps track of the original format,
by reading its header, and then restores the format when reimporting
it.
This way it also allows for having mixed styles.
Note: this version corrects a failure in the test
% files
dangling -> nothing
% tar
dangling -> nothing
% zip
dangling -> nothing