serve: return capability "value" from get_capability()
When the client sends v2 capabilities, they may be simple, like:
foo
or have a value like:
foo=bar
(all of the current capabilities actually expect a value, but the
protocol allows for boolean ones).
We use get_capability() to make sure the client's pktline matches a
capability. In doing so, we parse enough to see the "=" and the value
(if any), but we immediately forget it. Nobody cares for now, because they end
up parsing the values out later using has_capability(). But in
preparation for changing that, let's pass back a pointer so the callers
know what we found.
Note that unlike has_capability(), we'll return NULL for a "simple"
capability. Distinguishing these will be useful for some future patches.
Signed-off-by: Jeff King <peff@peff.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>