check_aliased_update: strcpy() instead of strcat() to copy

check_aliased_update: strcpy() instead of strcat() to copy

da3efdb (receive-pack: detect aliased updates which can occur with
symrefs, 2010-04-19) introduced two strcat() into uninitialized
strings.  The intent was clearly make a copy of the static buffer used
by find_unique_abbrev(), so use strcpy() instead.

Signed-off-by: Thomas Rast <trast@student.ethz.ch>
Reported-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Tested-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>