test-lib: add helper functions for config

There are a few common tasks when working with configuration variables in tests; this patch aims to make them a little easier to write and less error-prone. When setting a variable, you should typically make sure to clean it up after the test is finished, so as not to pollute other tests. Like: test_when_finished 'git config --unset foo.bar' && git config foo.bar baz This patch lets you just write: test_config foo.bar baz When clearing a variable that does not exist, git-config will report a specific non-zero error code. Meaning that tests which call "git config --unset" often either rely on the prior tests having actually set it, or must use test_might_fail. With this patch, the previous: test_might_fail git config --unset foo.bar becomes: test_unconfig foo.bar Not only is this easier to type, but it is more robust; it will correctly detect errors from git-config besides "key was not set". Signed-off-by: Jeff King <peff@peff.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>