teach 'update' cmd how to install hooks in bare repos

[technoweenie]

Instead of requiring git lfs update to be run inside a git working directory, this relies on lfs.LocalGitDir to install the hooks in the correct place.

/cc @ttaylorr since you touched hook code recently. Though I'm pretty sure I'm the one that added the invalid repo error.

[technoweenie]

Had to add GIT_LFS_TEST_DIR to travis. Some of the tests rely on commands being run outside of git repositories. By default, tests run inside the git-lfs repository, causing those to fail. Setting GIT_LFS_TEST_DIR outside of the git-lfs directory fixes it.

Ideally, the tests would warn and probably skip tests that should fail when this happens.

[technoweenie]

Confirmed this improves the process for migrating a repository to use LFS with BFG:

$ git clone --mirror https://github.com/mventurelli/relichunterszero
Cloning into bare repository 'relichunterszero.git'...

$ cd relichunterszero.git 
$ java -jar /path/to/bfg.jar --no-blob-protection --convert-to-git-lfs "*.{wav,mp3}"
$ git lfs init # sets up pre-push hook for the migrated repo

After that, set up the remote and push away!