config: fix a leak in git_config_copy_or_rename_section_in_file

In 52d59cc645 (branch: add a --copy (-c) option to go with --move (-m),
2017-06-18) a new strbuf variable was introduced, but not released.

Thus, when copying a branch that has any configuration, we have a
leak.

   $ git branch foo
   $ git config branch.foo.some-key some_value
   $ git branch -c foo bar

   Direct leak of 65 byte(s) in 1 object(s) allocated from:
       ... in xrealloc wrapper.c
       ... in strbuf_grow strbuf.c
       ... in strbuf_vaddf strbuf.c
       ... in strbuf_addf strbuf.c
       ... in store_create_section config.c
       ... in git_config_copy_or_rename_section_in_file config.c
       ... in git_config_copy_section_in_file config.c
       ... in git_config_copy_section config.c
       ... in copy_or_rename_branch builtin/branch.c
       ... in cmd_branch builtin/branch.c
       ... in run_builtin git.c

Let's fix that leak.

Signed-off-by: Rubén Justo <rjusto@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

config.c

@@ -3879,6 +3879,7 @@ out_no_rollback:
 	free(filename_buf);
 	config_store_data_clear(&store);
 	strbuf_release(&buf);
+	strbuf_release(&copystr);
 	return ret;
 }