multi(core): Updates to git-cache (#461)

feat(core): GitCache supports filtering lookups of ref and source info
feat(core): GitCache supports SHA-256 repos
fix(core): Fixed GitCache failure to get a parent directory after previously getting a descendant object
fix(core): GitCache no longer unnecessarily sets the write bit on files copied into a specified directory
fix(core): GitCache no longer cleans out existing files when writing to a shared source, which should reduce issues with concurrent clients

Author: dazuma