diff --no-index: reset temporary buffer lengths on directory iteration

diff --no-index: reset temporary buffer lengths on directory iteration

Commit 875b91b (diff --no-index: use strbuf for temporary pathnames,
2012-04-25) introduced a regression when using diff --no-index with
directories.  When iterating through a directory, the switch to strbuf
from heap-allocated char arrays caused paths to form like 'dir/file1',
'dir/file1file2', rather than 'dir/file1', 'dir/file2' as expected.

Avoid this by resetting the paths variables to their original length
before each iteration.

Signed-off-by: Bobby Powers <bobbypowers@gmail.com>
Reviewed-by: René Scharfe <rene.scharfe@lsrfire.ath.cx>
Signed-off-by: Junio C Hamano <gitster@pobox.com>