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>

t/test-lib.sh

@@ -357,6 +357,24 @@ test_chmod () {
 	git update-index --add "--chmod=$@"
 }
 
+# Unset a configuration variable, but don't fail if it doesn't exist.
+test_unconfig () {
+	git config --unset-all "$@"
+	config_status=$?
+	case "$config_status" in
+	5) # ok, nothing to unset
+		config_status=0
+		;;
+	esac
+	return $config_status
+}
+
+# Set git config, automatically unsetting it after the test is over.
+test_config () {
+	test_when_finished "test_unconfig '$1'" &&
+	git config "$@"
+}
+
 # Use test_set_prereq to tell that a particular prerequisite is available.
 # The prerequisite can later be checked for in two ways:
 #