Covers that multiple fragments actually works and the unique error
contidions, but otherwise relies on the lower level tests for
correctness. Also update some comments and the README based on
observations from writing/debugging these tests.
The parsed OIDs are usually prefixes, not full hashes, so rename the
fields appropriately. Also note that values that are too long to be
valid OIDs are still accepted by the library.
Fix parseGitHeaderIndex to allow abbreviated OIDs and add a note about
this to the README. The behavior might change again later, based on some
experimentation with Git.
Billy Keyes