submodule: use parse_config_key when parsing config

submodule: use parse_config_key when parsing config

This makes the code a lot simpler to read by dropping a
whole bunch of constant offsets.

As a bonus, it means we also feed the whole config variable
name to our error functions:

  [before]
  $ git -c submodule.foo.fetchrecursesubmodules=bogus checkout
  fatal: bad foo.fetchrecursesubmodules argument: bogus

  [after]
  $ git -c submodule.foo.fetchrecursesubmodules=bogus checkout
  fatal: bad submodule.foo.fetchrecursesubmodules argument: bogus

Signed-off-by: Jeff King <redacted>
Reviewed-by: Jonathan Nieder <redacted>
Acked-by: Jens Lehmann <redacted>
Signed-off-by: Junio C Hamano <redacted>