serve: reject bogus v2 "command=ls-refs=foo"

serve: reject bogus v2 "command=ls-refs=foo"

When we see a line from the client like "command=ls-refs", we parse
everything after the equals sign as a capability, which we check against
our capabilities table. If we don't recognize the command (e.g.,
"command=foo"), we'll reject it.

But in parse_command(), we use the same get_capability() parser for
parsing non-command lines. So if we see "command=ls-refs=foo", we will
feed "ls-refs=foo" to get_capability(), which will say "OK, that's
ls-refs, with value 'foo'". But then we simply ignore the value
entirely.

The client is violating the spec here, which says:

      command = PKT-LINE("command=" key LF)
      key = 1*(ALPHA | DIGIT | "-_")

I.e., the key is not even allowed to have an equals sign in it. Whereas
a real non-command capability does allow a value:

      capability = PKT-LINE(key[=value] LF)

So by reusing the same get_capability() parser, we are mixing up the
"key" and "capability" tokens. However, since that parser tells us
whether it saw an "=", we can still use it; we just need to reject any
input that produces a non-NULL value field.

The current behavior isn't really hurting anything (the client should
never send such a request, and if it does, we just ignore the "value"
part). But since it does violate the spec, let's tighten it up to
prevent any surprising behavior.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>