pathencode: fix hashmangle short dir limit (issue3958) The Python version of this (see mercurial/store.py:_hashencode) copies path components up to a limit of maxshortdirslen bytes. The Python version does not consider the initial "dh/" to be part of the this, though, while the C version currently does. Adding len("dh/") == 3 to the limit for the C version brings it in line with the Python version. This was not caught by the randomized testing scheme in test-pathencode.py because of a couple of flaws with the test. Upcoming patches will fix those problems.