Relative paths to the home folder are failing.
Both absolute and relative paths (to the CWD) appear to be working correctly.

Example:
```
testrun_addons --with-untrusted --repository=~/work/mozilla/bugs/927397/mozmill-tests --report=http://mozmill-crowd.blargon7.com/db/ /Applications/FirefoxNightly.app/ --workspace=~/workspace/
```

Fails with:
```
CalledProcessError: Command '['hg', 'clone', '~/work/mozilla/bugs/927397/mozmill-tests', '/Users/andrei.eftimie/work/mozilla/mozmill-automation/src/~/workspace/mozmill-tests', '--cwd', '/Users/andrei.eftimie/work/mozilla/mozmill-automation/src']' returned non-zero exit status 255
```