We currently do not handle those enum values which require us to set
"true" or unset variables in all cases. Use a common function which does
understand this by looking at our mapping directly.

Having this cache and giving them out goes against our multithreading
guarantees and it makes it impossible to use submodules in a
multi-threaded environment, as any thread can ask for a refresh which
may reallocate some string in the submodule struct which we've accessed
in a different one via a getter.

This makes the submodules behave more like remotes, where each object is
created upon request and not shared except explicitly by the user. This
means that some tests won't pass yet, as they assume they can affect the
submodule objects in the cache and that will affect later operations.

This is used by the submodule in order to figure out if the index has
changed since it last read it. Using a timestamp is racy, so let's make
it use the checksum, just like we now do for reloading the index itself.

This makes submodule cache refresh actually look at the timestamps
from the data sources for submodules and reload as needed if they
have changed since the last refresh.

This takes the old submodule cache which was just a git_strmap
and makes a real git_submodule_cache object that can contain other
things like a lock and timestamp-ish data to control refreshing of
submodule info.

This cleans up some places I missed that could hold onto submodule
references and cleans up the way in which the repository cache is
both reloaded and released so that existing submodule references
aren't destroyed inappropriately.

`git_submodule` objects were already refcounted internally in case
the submodule name was different from the path at which it was
stored.  This makes that refcounting externally used as well, so
`git_submodule_lookup` and `git_submodule_add_setup` return an
object that requires a `git_submodule_free` when done.

This removes the fetchRecurse compiler warnings and makes the
behavior match the other submodule options (i.e. the in-memory
setting can be reset to the on-disk value).