t4202: use test_config/test_unconfig to set/unset git config variables

Instead of using construct such as:
    test_when_finished "git config --unset <key>"
    git config <key> <value>
uses
    test_config <key> <value>
The latter takes care of removing <key> at the end of the test.

Additionally, instead of
     git config <key> ""
or
     git config --unset <key>
uses
     test_unconfig <key>
The latter doesn't failed if <key> is not defined.

Tests are modified to assume correct (default) configuration at entry,
and to reset the modified configuration variables at the end.

Signed-off-by: Yann Droneaud <ydroneaud@opteya.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t4202-log.sh

@@ -419,8 +419,6 @@ test_expect_success 'log --graph with merge' '
 '
 
 test_expect_success 'log.decorate configuration' '
-	test_might_fail git config --unset-all log.decorate &&
-
 	git log --oneline >expect.none &&
 	git log --oneline --decorate >expect.short &&
 	git log --oneline --decorate=full >expect.full &&
@@ -429,61 +427,55 @@ test_expect_success 'log.decorate configuration' '
 	git log --oneline >actual &&
 	test_cmp expect.short actual &&
 
-	git config --unset-all log.decorate &&
-	git config log.decorate true &&
+	test_config log.decorate true &&
 	git log --oneline >actual &&
 	test_cmp expect.short actual &&
 	git log --oneline --decorate=full >actual &&
 	test_cmp expect.full actual &&
 	git log --oneline --decorate=no >actual &&
 	test_cmp expect.none actual &&
 
-	git config --unset-all log.decorate &&
-	git config log.decorate no &&
+	test_config log.decorate no &&
 	git log --oneline >actual &&
 	test_cmp expect.none actual &&
 	git log --oneline --decorate >actual &&
 	test_cmp expect.short actual &&
 	git log --oneline --decorate=full >actual &&
 	test_cmp expect.full actual &&
 
-	git config --unset-all log.decorate &&
-	git config log.decorate 1 &&
+	test_config log.decorate 1 &&
 	git log --oneline >actual &&
 	test_cmp expect.short actual &&
 	git log --oneline --decorate=full >actual &&
 	test_cmp expect.full actual &&
 	git log --oneline --decorate=no >actual &&
 	test_cmp expect.none actual &&
 
-	git config --unset-all log.decorate &&
-	git config log.decorate short &&
+	test_config log.decorate short &&
 	git log --oneline >actual &&
 	test_cmp expect.short actual &&
 	git log --oneline --no-decorate >actual &&
 	test_cmp expect.none actual &&
 	git log --oneline --decorate=full >actual &&
 	test_cmp expect.full actual &&
 
-	git config --unset-all log.decorate &&
-	git config log.decorate full &&
+	test_config log.decorate full &&
 	git log --oneline >actual &&
 	test_cmp expect.full actual &&
 	git log --oneline --no-decorate >actual &&
 	test_cmp expect.none actual &&
 	git log --oneline --decorate >actual &&
 	test_cmp expect.short actual
 
-	git config --unset-all log.decorate &&
+	test_unconfig log.decorate &&
 	git log --pretty=raw >expect.raw &&
-	git config log.decorate full &&
+	test_config log.decorate full &&
 	git log --pretty=raw >actual &&
 	test_cmp expect.raw actual
 
 '
 
 test_expect_success 'reflog is expected format' '
-	test_might_fail git config --remove-section log &&
 	git log -g --abbrev-commit --pretty=oneline >expect &&
 	git reflog >actual &&
 	test_cmp expect actual
@@ -496,10 +488,6 @@ test_expect_success 'whatchanged is expected format' '
 '
 
 test_expect_success 'log.abbrevCommit configuration' '
-	test_when_finished "git config --unset log.abbrevCommit" &&
-
-	test_might_fail git config --unset log.abbrevCommit &&
-
 	git log --abbrev-commit >expect.log.abbrev &&
 	git log --no-abbrev-commit >expect.log.full &&
 	git log --pretty=raw >expect.log.raw &&
@@ -508,7 +496,7 @@ test_expect_success 'log.abbrevCommit configuration' '
 	git whatchanged --abbrev-commit >expect.whatchanged.abbrev &&
 	git whatchanged --no-abbrev-commit >expect.whatchanged.full &&
 
-	git config log.abbrevCommit true &&
+	test_config log.abbrevCommit true &&
 
 	git log >actual &&
 	test_cmp expect.log.abbrev actual &&