extdiff: remove dir2root and pass full path as dir2 in _runperfilediff()
The only use of `dir2root` was to join with `dir2` to generate the path for
other side of diff. Like in previous patch, `dir1a` and `dir1b` are full paths
and no longer base names, hence we pass `dir2` as full path too and making
`dir2root` unrequired.
Differential Revision: https://phab.mercurial-scm.org/D8970
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