mq: some improvements to header rewriting (closes #690)
Tested with qrefresh -m against the mm series.
% should failabort: path contains illegal component: .hg/00changelog.iadding a/a% should failabort: path 'b/b' traverses symbolic link 'b'% should succeed% should still fail - maybeabort: path 'b/b' traverses symbolic link 'b'