tests: refactor a few tests to use "test_commit --append"

Refactor a few more tests to use the new "--append" option to
"test_commit". I added it for use in the mailmap tests, but this
demonstrates how useful it is in general.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t1412-reflog-loop.sh

@@ -4,11 +4,8 @@ test_description='reflog walk shows repeated commits again'
 . ./test-lib.sh
 
 test_expect_success 'setup commits' '
-	test_tick &&
-	echo content >file && git add file && git commit -m one &&
-	git tag one &&
-	echo content >>file && git add file && git commit -m two &&
-	git tag two
+	test_commit one file content &&
+	test_commit --append two file content
 '
 
 test_expect_success 'setup reflog with alternating commits' '

t/t2012-checkout-last.sh

@@ -5,13 +5,9 @@ test_description='checkout can switch to last branch and merge base'
 . ./test-lib.sh
 
 test_expect_success 'setup' '
-	echo hello >world &&
-	git add world &&
-	git commit -m initial &&
+	test_commit initial world hello &&
 	git branch other &&
-	echo "hello again" >>world &&
-	git add world &&
-	git commit -m second
+	test_commit --append second world "hello again"
 '
 
 test_expect_success '"checkout -" does not work initially' '
@@ -93,9 +89,7 @@ test_expect_success 'switch to twelfth from the last' '
 
 test_expect_success 'merge base test setup' '
 	git checkout -b another other &&
-	echo "hello again" >>world &&
-	git add world &&
-	git commit -m third
+	test_commit --append third world "hello again"
 '
 
 test_expect_success 'another...master' '

t/t7810-grep.sh

@@ -687,21 +687,9 @@ test_expect_success 'grep -C1 hunk mark between files' '
 '
 
 test_expect_success 'log grep setup' '
-	echo a >>file &&
-	test_tick &&
-	GIT_AUTHOR_NAME="With * Asterisk" \
-	GIT_AUTHOR_EMAIL="xyzzy@frotz.com" \
-	git commit -a -m "second" &&
-
-	echo a >>file &&
-	test_tick &&
-	git commit -a -m "third" &&
-
-	echo a >>file &&
-	test_tick &&
-	GIT_AUTHOR_NAME="Night Fall" \
-	GIT_AUTHOR_EMAIL="nitfol@frobozz.com" \
-	git commit -a -m "fourth"
+	test_commit --append --author "With * Asterisk <xyzzy@frotz.com>" second file a &&
+	test_commit --append third file a &&
+	test_commit --append --author "Night Fall <nitfol@frobozz.com>" fourth file a
 '
 
 test_expect_success 'log grep (1)' '