This adds a basic test of doing simultaneous diffs on multiple
threads and adds basic locking for the attr file cache because
that was the immediate problem that arose from these tests.
This step is needed to easily add iterators to git_config_backend
As well use these new git_strmap functions to implement foreach
* git_strmap_iter
* git_strmap_has_data(...)
* git_strmap_begin(...)
* git_strmap_end(...)
* git_strmap_next(...)
This renamed `git_khash_str` to `git_strmap`, `git_hash_oid` to
`git_oidmap`, and deletes `git_hashtable` from the tree, plus
adds unit tests for `git_strmap`.
