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

hgext/extdiff.py

@@ -255,7 +255,6 b' def _runperfilediff('
     tmproot,
     dir1a,
     dir1b,
-    dir2root,
     dir2,
     rev1a,
     rev1b,
@@ -280,7 +279,7 b' def _runperfilediff('
             if not os.path.isfile(path1b):
                 path1b = pycompat.osdevnull
 
-        path2 = os.path.join(dir2root, dir2, commonfile)
+        path2 = os.path.join(dir2, commonfile)
         label2 = commonfile + rev2
 
         if confirm:
@@ -501,8 +500,7 b' def diffrevs('
             tmproot=tmproot,
             dir1a=os.path.join(tmproot, dir1a),
             dir1b=os.path.join(tmproot, dir1b) if do3way else None,
-            dir2root=dir2root,
-            dir2=dir2,
+            dir2=os.path.join(dir2root, dir2),
             rev1a=rev1a,
             rev1b=rev1b,
             rev2=rev2,