store: optimize _pathencode by checking the length of the unencoded path

If the input path is already longer than _maxstorepathlen, then we can skip
doing the basic encoding (encodedir, _encodefname and _auxencode) and directly
proceed to the hashed encoding. Those encodings, if at all, will make the path
only longer.

mercurial/store.py

@@ -255,6 +255,8 b' def _hybridencode(path, dotencode):'
     return res
 
 def _pathencode(path):
+    if len(path) > _maxstorepathlen:
+        return None
     ef = _encodefname(encodedir(path)).split('/')
     res = '/'.join(_auxencode(ef, True))
     if len(res) > _maxstorepathlen: