Don't build unconfigured branches

[sbeitzel]

When a commit happens on a branch that hasn't been set up on the build server, the build server still tries to build the branch. However, because the directory doesn't exist, the script fails. This will prevent other commits from getting built.

Example error log:

Mar 31 15:25:32 ip-172-30-0-31 triggercheck[11868]: changed directory to /var/zuleika/repos/g4_march_in_off
Mar 31 15:25:32 ip-172-30-0-31 triggercheck[11868]: fatal: not a git repository (or any of the parent directories): .git
Mar 31 15:25:32 ip-172-30-0-31 triggercheck[11868]: make: *** No rule to make target 'sync'.  Stop.
Mar 31 15:25:32 ip-172-30-0-31 triggercheck[11868]: Use of uninitialized value $message in concatenation (.) or string at /var/zuleika/mtbin/triggercheck line 58.
Mar 31 15:25:32 ip-172-30-0-31 systemd[1]: triggercheck.service: Main process exited, code=exited, status=2/INVALIDARGUMENT
Mar 31 15:25:32 ip-172-30-0-31 systemd[1]: triggercheck.service: Failed with result 'exit-code'.
Mar 31 15:25:32 ip-172-30-0-31 systemd[1]: Failed to start Checks for change notification trigger files. Part of the music tools pipeline..