Instrument git_submodule_set_url.

This commit adds a setter for the url property of the Submodule class.
The new setter leverages 'git_submodule_set_url' from libgit2.

Author: nrybowski

pygit2/decl/submodule.h

@@ -41,3 +41,5 @@ const char * git_submodule_branch(git_submodule *submodule);
 const git_oid * git_submodule_head_id(git_submodule *submodule);
 
 int git_submodule_status(unsigned int *status, git_repository *repo, const char *name, git_submodule_ignore_t ignore);
+
+int git_submodule_set_url(git_repository *repo, const char *name, const char *url);

pygit2/submodules.py

@@ -152,6 +152,14 @@ def url(self) -> Union[str, None]:
         url = C.git_submodule_url(self._subm)
         return maybe_string(url)
 
+    @url.setter
+    def url(self, url: str) -> None:
+        crepo = self._repo._repo
+        cname = ffi.new('char[]', to_bytes(self.name))
+        curl = ffi.new('char[]', to_bytes(url))
+        err = C.git_submodule_set_url(crepo, cname, curl)
+        check_error(err)
+
     @property
     def branch(self):
         """Branch that is to be tracked by the submodule."""