xmkstemp(): avoid showing truncated template more carefully

xmkstemp(): avoid showing truncated template more carefully

Some implementations of xmkstemp() leaves the given in/out buffer
truncated when they return with failure.

6cf6bb3 (Improve error messages when temporary file creation fails,
2010-12-18) attempted to show the real filename we tried to create
(but failed), and if that is not available due to such truncation,
to show the original template that was given by the caller.

But it failed to take into account that the given template could
have "directory/" in front, in which case the truncation point may
not be template[0] but somewhere else.

Signed-off-by: Junio C Hamano <gitster@pobox.com>