t4018 (funcname patterns): make configuration easier to track

t4018 (funcname patterns): make configuration easier to track

Introduce a "test_config" function to set a configuration variable
for use by a single test (automatically unsetting it when the
assertion finishes).  If this function is used consistently, the
configuration used in a test_expect_success block can be read at the
beginning of that block instead of requiring reading all the tests
that come before.  So it becomes a little easier to add new tests or
rearrange existing ones without fear of breaking configuration.

In particular, the test of alternation in xfuncname patterns also
checks that xfuncname takes precedence over funcname variable as a
sort of side-effect, since the latter leaks in from previous tests.
In the new syntax, the test has to say explicitly what variables it is
using, making the test clearer and a future regression in coverage
from carelessly editing the script less likely.

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