t7003: add test to filter a branch with a commit at epoch

Running filter-branch on a history that has a commit with timestamp
at epoch used to fail, but it should have been fixed.  Add test to
make sure it won't break again.
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

t/t7003-filter-branch.sh

@@ -5,7 +5,8 @@ test_description='git filter-branch'
 
 test_expect_success 'setup' '
 	test_commit A &&
-	test_commit B &&
+	GIT_COMMITTER_DATE="@0 +0000" GIT_AUTHOR_DATE="@0 +0000" &&
+	test_commit --notick B &&
 	git checkout -b branch B &&
 	test_commit D &&
 	mkdir dir &&

t/test-lib.sh

@@ -335,10 +335,19 @@ test_tick () {
 # Both <file> and <contents> default to <message>.
 
 test_commit () {
-	file=${2:-"$1.t"}
+	notick= &&
+	if test "z$1" = "z--notick"
+	then
+		notick=yes
+		shift
+	fi &&
+	file=${2:-"$1.t"} &&
 	echo "${3-$1}" > "$file" &&
 	git add "$file" &&
-	test_tick &&
+	if test -z "$notick"
+	then
+		test_tick
+	fi &&
 	git commit -m "$1" &&
 	git tag "$1"
 }