builtin/push.c: remove useless temporary variable

builtin/push.c: remove useless temporary variable

Creating a variable nr here to use throughout the function only to change
refspec_nr to nr at the end, having not used refspec_nr the entire time,
is rather pointless. Instead, simply increment refspec_nr.

While at it, use ALLOC_GROW() instead of xrealloc().

Signed-off-by: Jared Hance <jaredhance@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>