rebase: fix path auditing to audit path relative to repo root (issue5818)

Authored by martinvonz.

Description

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