Changed \$callsign to \$argv[1] on commit_hook.php since is undefined causing an error when trying to report an error.

Summary:
Phabricator was going to give me an error message via commit_hook.php, unfortunately said error wasn't being set since
\$callsign was undefined. So, just changed \$callsign to \$argv[1] and now I get the appropriate commit.

Test Plan:

1. Add commit_hook.php to an SVN pre-commit.
2. Set the SVN to be hosted off of Phabricator.
3. Attempt to commit to commit to SVN repository.

Expected: Error message saying that the repository isn't hosted on Phabricator
Results: Error message saying undefined function.

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley

CC: Korvin, epriestley, aran

Differential Revision: https://secure.phabricator.com/D7920