scmutil: speed up relativization of paths when it's a no-op

Running commands from the root is commmon, in particular for
automation. Running `hg files > /tmp/a` from the root of
mozilla-central on linux:

before:
real	0m1,510s
user	0m1,387s
sys	0m0,090s

after:
real	0m1,266s
user	0m1,165s
sys	0m0,073s

(there are 280k paths, so this was costing ~1us per path somehow)

Differential Revision: https://phab.mercurial-scm.org/D8585

mercurial/scmutil.py

@@ -803,9 +803,12 b' def getuipathfn(repo, legacyrelativevalu'
 
     if relative:
         cwd = repo.getcwd()
-        pathto = repo.pathto
-        return lambda f: pathto(f, cwd)
-    elif repo.ui.configbool(b'ui', b'slash'):
+        if cwd != b'':
+            # this branch is correct when cwd == b'', ie cwd = repo root,
+            # but it's slower
+            pathto = repo.pathto
+            return lambda f: pathto(f, cwd)
+    if repo.ui.configbool(b'ui', b'slash'):
         return lambda f: f
     else:
         return util.localpath