vcs: Fix internal server error when trying to get diff from Mercurial for paths that include globbing patterns (Issue #308): - Treat passed-in paths as exact matches, dissallowing any type of globbing/regex/pattern matching. - Added accompanying tests.

[tox]
minversion = 1.8
envlist = py{26,27}-{pytest,nose}

[testenv]
setenv =
    PYTHONHASHSEED = 0
deps =
    nose: nose
    pytest: pytest
commands =
    nose: nosetests {posargs}
    pytest: py.test {posargs}