env: remove the _from_env function

Remove the `_git_repository_open_ext_from_env` and make it part of the
general repository opening code path. This removes the re-entrancy, and
standardizes things like index and config opening to a single place
again so that we have predictable ordering of the opening procedure.

src/libgit2/repository.h

@@ -151,8 +151,9 @@ struct git_repository {
 
 	git_array_t(git_str) reserved_names;
 
-	unsigned is_bare:1;
-	unsigned is_worktree:1;
+	unsigned use_env:1,
+	         is_bare:1,
+	         is_worktree:1;
 	git_oid_t oid_type;
 
 	unsigned int lru_counter;