traverse_trees(): clarify return value of the callback

traverse_trees(): clarify return value of the callback

The variable name "ret" sounds like the variable to be returned, but
since e6c111b4 we return error, and it is misleading.

As this variable tells us which trees in t[] array were used in the
callback function, so that this caller can know the entries in which
of the trees need advancing, "trees_used" is a better name.

Also the assignment to 0 was removed at the start of the function as
well after the "if (interesting)" block.  Those are unneeded as that
variable is set to the callback return value any time we enter the
"if (interesting)" block, so we'd overwrite old values anyway.

Signed-off-by: Stefan Beller <stefanbeller@googlemail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>