Fix mkpath abuse in dwim_ref and dwim_log of sha1_name.c

Fix mkpath abuse in dwim_ref and dwim_log of sha1_name.c

Otherwise the function sometimes fail to resolve obviously correct
refnames, because the string data pointed to by "str" argument were
reused.

The change in dwim_log does not fix anything, just optimizes away
strcpy code as the path can be created directly in the available
buffer.

Signed-off-by: Alex Riesen <raa.lkml@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>