Supress git rev-parse error when fetching Git revision

[shalvah]

Pipe stderr from the git rev-parse HEAD to the null device (/dev/null on *nix, NUL on Windows). Also replaced references to /dev/null in the tests with Ruby's FIle::NULL (available since Ruby 1.9), so tests can work on Windows also.

[shalvah]

Tested in these situations:

• No Git repo: from_git returns nil
• Git repo but not committed: from_git returns "HEAD"
• Git repo with commits: from_git returns lastest commit

[joshuap]

Nice! 👍