fix "git -c" parsing of values with equals signs

fix "git -c" parsing of values with equals signs

If you do something like:

  git -c core.foo="value with = in it" ...

we would split your option on "=" into three fields and
throw away the third one. With this patch we correctly take
everything after the first "=" as the value (keys cannot
have an equals sign in them, so the parsing is unambiguous).

Signed-off-by: Jeff King <redacted>
Signed-off-by: Junio C Hamano <redacted>