diff-highlight: use test_tick in graph test

The exact ordering output by Git may depend on the commit
timestamps, so let's make sure they're actually
monotonically increasing, and not all the same (or worse,
subject to how long the test script takes to run).

Let's use test_tick to make sure this is stable. Note that
we actually have to rearrange the order of the branches to
match the expected graph structure (which means that
previously we might racily have been testing a slightly
different output, though the test is written in such a way
that we'd still pass).

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

contrib/diff-highlight/t/t9400-diff-highlight.sh

@@ -52,9 +52,9 @@ test_strip_patch_header () {
 # dh_test_setup_history generates a contrived graph such that we have at least
 # 1 nesting (E) and 2 nestings (F).
 #
-#	  A branch
+#	  A master
 #	 /
-#	D---E---F master
+#	D---E---F branch
 #
 #	git log --all --graph
 #	* commit
@@ -74,18 +74,22 @@ dh_test_setup_history () {
 
 	cat file1 >file &&
 	git add file &&
+	test_tick &&
 	git commit -m "D" &&
 
 	git checkout -b branch &&
 	cat file2 >file &&
-	git commit -a -m "A" &&
-
-	git checkout master &&
-	cat file2 >file &&
+	test_tick &&
 	git commit -a -m "E" &&
 
 	cat file3 >file &&
-	git commit -a -m "F"
+	test_tick &&
+	git commit -a -m "F" &&
+
+	git checkout master &&
+	cat file2 >file &&
+	test_tick &&
+	git commit -a -m "A"
 }
 
 left_trim () {