Implemented FITS execution.
Tidied up release unzipping.
Multiple changes to main fits-test script.

 * Added check for fresh branch with no commits.
 * Added function to checkout particular git revision.
 * Added function to find the FITS release directory.
 * Implemented FITS execution.
 * Improved comments.

The script now:

 * Establishes the git revisions to compare.
 * Builds the current HEAD revision.
 * Runs HEAD FITS output to ./.output/[HEAD hash].
 * Checks out and builds the branch root master revision.
 * Runs root FITS output to ./.output/[root hash].