remote: use parse_config_key

remote: use parse_config_key

95b567c7 ("use skip_prefix to avoid repeating strings") transformed
calls using starts_with() and then skipping the length of the prefix to
skip_prefix() calls.  In remote.c there are a few calls like:

  if (starts_with(foo, "bar"))
      foo += 3

These calls weren't touched by the aformentioned commit, but can be
replaced by calls to parse_config_key(), to simplify the code and
clarify the intentions.  Do that.

Helped-by: Jeff King <peff@peff.net>
Signed-off-by: Thomas Gummerer <t.gummerer@gmail.com>
Reviewed-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>