util: use bytes re on bytes input in fspath

Fixes `hg add` on Python 3.

mercurial/util.py

@@ -1331,7 +1331,7 b' def fspath(name, root):'
         seps = seps + pycompat.osaltsep
     # Protect backslashes. This gets silly very quickly.
     seps.replace('\\','\\\\')
-    pattern = remod.compile(r'([^%s]+)|([%s]+)' % (seps, seps))
+    pattern = remod.compile(br'([^%s]+)|([%s]+)' % (seps, seps))
     dir = os.path.normpath(root)
     result = []
     for part, sep in pattern.findall(name):