fix segfault with git log -c --follow

fix segfault with git log -c --follow

In diff_tree_combined we make a copy of diffopts. In
try_to_follow_renames, called via diff_tree_sha1, we free and
re-initialize diffopts->pathspec->items. Since we did not make a deep
copy of diffopts in diff_tree_combined, the original diffopts does not
get the update. By the time we return from diff_tree_combined,
rev->diffopt->pathspec->items points to an invalid memory address. We
get a segfault next time we try to access that pathspec.

Instead, along with the copy of diffopts, make a copy pathspec->items as
well.

We would also have to make a copy of pathspec->raw to keep it consistent
with pathspec->items, but nobody seems to rely on that.

Signed-off-by: Clemens Buchacher <drizzd@aon.at>
Signed-off-by: Junio C Hamano <gitster@pobox.com>