When a submodule was inserted with a different path and name, the
return value from khash greater than zero was allowed to propagate
back out to the caller when it should really be zeroed. This led
to a possible crash when reloading submodules if that was the
first time that submodule data was loaded.
`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.
Any well-behaved program should write a descriptive message to the
reflog whenever it updates a reference. Let's make this more prominent
by removing the version without the reflog parameters.
