This adds a bunch of additional config values to the repository
config value cache and makes it easier to add a simple boolean
config without creating enum values for each possible setting.
Also, this fixes a bug in git_config_refresh where the config
cache was not being cleared which could lead to potential
incorrect values.
The work to start using the new cached configs will come in the
next couple of commits...
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`.
