remote-curl: create copy of the service name

remote-curl: create copy of the service name

Make a copy of the service name being requested instead of relying on
the buffer pointed to by the passed in 'const char *' to remain
unchanged.

Currently, all service names are string constants, but a subsequent
patch will introduce service names from external sources.

Signed-off-by: Brandon Williams <bmwill@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>