repository: clear out-parameter instead of freeing it

The `path` out-parameter of `find_repo` is being sanitized initially
such that we do not try to append to existing content. The sanitization
is done via `git_buf_free`, though, which forces us to needlessly
reallocate the buffer later in the function. Fix this by using
`git_buf_clear` instead.

src/repository.c

@@ -440,7 +440,7 @@ static int find_repo(
 	bool in_dot_git;
 	size_t ceiling_offset = 0;
 
-	git_buf_free(repo_path);
+	git_buf_clear(repo_path);
 
 	error = git_path_prettify(&path, start_path, NULL);
 	if (error < 0)