diffcore-rename: make diff-tree -l0 mean -l<large>

diffcore-rename: make diff-tree -l0 mean -l<large>

In the documentation of diff-tree, it is stated that the -l option
"prevents rename/copy detection from running if the number of
rename/copy targets exceeds the specified number". The documentation
does not mention any special handling for the number 0, but the
implementation before commit 9f7e4bfa3b ("diff: remove silent clamp of
renameLimit", 2017-11-13) treated 0 as a special value indicating that
the rename limit is to be a very large number instead.

The commit 9f7e4bfa3b changed that behavior, treating 0 as 0. Revert
this behavior to what it was previously. This allows existing scripts
and tools that use "-l0" to continue working. The alternative (to have
"-l0" suppress rename detection) is probably much less useful, since
users can just refrain from specifying -M and/or -C to have the same
effect.

Signed-off-by: Jonathan Tan <jonathantanmy@google.com>
Reviewed-by: Jonathan Nieder <jrnieder@gmail.com>
Reviewed-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>