Fail with a clear error if 'git fat pull' is run on uninitialised repo.

This fixes issue #25 in the original project.
Make test-restrospective.sh test script compatible with OSX
Moved requirement for init to `checkout` operation.

Since `checkout` op is where the repo really needs to be init'ed for git-fat
I moved the requirement to there. This way it is still triggered by a `pull`
op, but only at the end after any files have been synced.
Add `verify` command to check git-fat object data matches hash filename.

While experimenting with using the --partial option with rsync I
managed to corrupt one of my git-fat objects by truncating it. This
caused some behaviour in git-fat which seemed odd until I worked out
what had happened: it would check out the truncated data but git would
see the file as modified and show the changed hash in a diff, while a
re-checkout did not reset the file to its original data/hash.

This commit adds a `verify` command that cross-checks git-fat object
file names (the original SHA1) against the SHA1 of the object's
actual data and prints any mismatches. So you can quickly find any
dubious objects and decide what to do about them.

A better solution might be to calcuate and verify objects' data hash
during filter-smudge/checkout though this would likely hurt performance.
Print fatal error message to stderr
Merge branch 'jmurty/ensure-init-before-pull' (PR #33)

* jmurty/ensure-init-before-pull:
  Print fatal error message to stderr
  Moved requirement for init to `checkout` operation.
  Fail with a clear error if 'git fat pull' is run on uninitialised repo.
Merge branch 'add_verify_command' of github:jmurty/git-fat (PR #36)

* 'add_verify_command' of github:jmurty/git-fat:
  Add `verify` command to check git-fat object data matches hash filename.

Conflicts:
	git-fat
Move init diagnostics out of cmd_* section
Replace heavyweight use of python with a simple 'pwd' call
Use custom fullpath() fn to avoid confusion with realpath vs alternative

* Define and use simple `pwd`-based fullpath() function
* Remove misleading comment

Confirmed to work on OSX.
Merge branch 'osx_test_compat_fixes' of github:jmurty/git-fat (PR #35)

* 'osx_test_compat_fixes' of github:jmurty/git-fat:
  Use custom fullpath() fn to avoid confusion with realpath vs alternative
  Replace heavyweight use of python with a simple 'pwd' call
  Make test-restrospective.sh test script compatible with OSX
Merge branch 'upstream' into improve_referenced_objects_performance
Print filenames for objects in `status` output with '--filenames' option

The '--filenames' option makes it easy to view the filename represented
by a git-fat object reference, at the expense of a performance hit
compared to the plain `git-fat status` command. So you can know what you
are about to delete before you run `git-fat gc`...

* Extended referenced_objects() lookup to build a map of
  git-fat-object => githashes by default behind the scenes
* Added method to lookup filenames for a given set of git hashes
* If '--filenames' option is given to status command, collect all git
  hash values corresponding to git-fat objects to be printed, and lookup
  their filenames.
* Print filename next to object hash value with space delimiter