Added the ability to create a ref (much like a git tag) for an
existing commit through "ostree refs EXISTING --create=NEWREF".
Previously the only way to create a new ref was by creating a new commit,
but refs --create allows multiple refs to point to the same commit.
The command will fail if:
- None/more than one existing ref is specified
- The specified EXISTING tag does not exist, or was not specified
- The specified NEWREF already exists, or is the name of a folder
Add unit tests in tests-ref.sh to verify above functionality
Closes: #340
Approved by: jlebon
