refspec: make sure stack refspec_item variables are zeroed

refspec: make sure stack refspec_item variables are zeroed

A couple of functions that used struct refspec_item did not zero out the
structure memory. This can result in unexpected behavior, especially if
additional parameters are ever added to refspec_item in the future. Use
memset to ensure that unset structure members are zero.

It may make sense to convert most of these uses of struct refspec_item
to use either struct initializers or refspec_item_init_or_die. However,
other similar code uses memset. Converting all of these uses has been
left as a future exercise.

Signed-off-by: Jacob Keller <redacted>
Signed-off-by: Junio C Hamano <redacted>