It must be non-NULL to have a valid `git_indexer`.

Encapsulation!

Submodule optimization

Only try to `unlink` our temp file when we know that we didn't copy it
into its permanent location.

`git_submodule_status` is very slow, bottlenecked on
`git_repository_head_tree`, which it uses through `submodule_update_head`.  If
the user has requested submodule caching, assume that they want this status
cached too and skip it.

Signed-off-by: David Turner <dturner@twosigma.com>

Added `git_repository_submodule_cache_all` to initialze a cache of
submodules on the repository so that operations looking up N
submodules are O(N) and not O(N^2).  Added a
`git_repository_submodule_cache_clear` function to remove the cache.

Also optimized the function that loads all submodules as it was itself
O(N^2) w.r.t the number of submodules, having to loop through the
`.gitmodules` file once per submodule.  I changed it to process the
`.gitmodules` file once, into a map.

Signed-off-by: David Turner <dturner@twosigma.com>

Signed-off-by: David Turner <dturner@twosigma.com>

WinHTTP: support best auth mechanism

For username/password credentials, support NTLM or Basic (in that order
of priority).  Use the WinHTTP built-in authentication support for both,
and maintain a bitfield of the supported mechanisms from the response.