tests: check git does not barf on merge.ff values for future versions of git

tests: check git does not barf on merge.ff values for future versions of git

Maybe some day in the future we will want to support a syntax
like

	[merge]
		ff = branch1
		ff = branch2
		ff = branch3

in addition to the currently permitted "true", "false", and "only"
values.  So make sure we continue to treat such configurations as
though an unknown variable had been defined rather than erroring out,
until it is time to implement such a thing, so configuration files
using such a facility can be shared between present and future git.

While at it, add a few missing && and start the "combining --squash
and --no-ff" test with a known state so we can be sure it does not
succeed or fail for the wrong reason.

Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>