tests: apply: fix reference to deprecated `git_buf_free`

Since commit 56ffdfc6 (buffer: deprecate `git_buf_free` in favor of
`git_buf_dispose`, 2018-02-08), the function `git_buf_free` is
deprecated and shall not be used anymore. As part of the new apply
framework that has been cooking for quite some time some new references
have been introduced to that deprecated function. Replace them with
calls to `git_buf_dispose`.

tests/apply/partial.c

@@ -104,8 +104,8 @@ static int apply_buf(
 	}
 
 	git__free(filename);
-	git_buf_free(&result);
-	git_buf_free(&patchbuf);
+	git_buf_dispose(&result);
+	git_buf_dispose(&patchbuf);
 	git_patch_free(patch);
 
 	return error;