t4018 (funcname patterns): make .gitattributes state easier to track

t4018 (funcname patterns): make .gitattributes state easier to track

Most, but not all, tests in this script rely on attributes declaring
that files with a .java extension should use the "java" driver:

	*.java diff=java

Split out a "set up" test to put such a .gitattributes in place after
the tests that do not want it have run, to make it more likely that
individual tests other than this setup test can be safely modified,
rearranged, or skipped.  Presumably this setup code will learn to
request other drivers for other extensions in the same place when the
test suite learns to exercise other diff drivers.

Similarly, make sure that early test assertions that do not use these
default attributes set up .gitattributes appropriately for themselves,
so tests that run before can be modified with less risk of breaking
something.

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