Ensure consistent output from git command

The git command output can vary by language which would cause assertions
errors when parsing the output.

On POSIX system the language used by git can be adjusted by LC_MESSAGES.
The special language 'C' is guaranteed to be always available and is
whatever default the software has been written in (usually english, the
case for git).

Thus passing LC_MESSAGES to Popen will ensure we receive from git a
consistent output regardless of the user preference.

Addresses #153
Merge pull request #181 from hashar/git-exec-const-output

Ensure consistent output from git command