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].
Added reset HEAD --hard to revert to start state.
Stopped echoing FITS output.
Initial test execution, tested against an always succeed mock.
Added rm output dir to prevent accumulation of test data.
Merge branch 'invoke-test-tool' into cli_dev