rebase: fix path auditing to audit path relative to repo root (issue5818) Before this patch, when rebasing a file called "foo/bar", we would check e.g. if "/foo" (i.e. rooted at the file system root) was a symlink. Differential Revision: https://phab.mercurial-scm.org/D5361

pager.pager=p0 # $TESTTMP/sysrc:4
ui.editor=e1 # $TESTTMP/userrc:2

pager.pager=p2 # $PAGER
ui.editor=e1 # $TESTTMP/userrc:2