t7814: do not generate same commits in different repos

t7814 has repo tree like this

  initial-repo
    submodule
      sub

In each repo 'submodule' and 'sub', a commit is made to add the same
initial file 'a' with the same message 'add a'. If tests run fast
enough, the two commits are made in the same second, resulting
identical commits.

There is nothing wrong with that per-se. But it could make the test
flaky. Currently all submodule odbs are merged back in the main
one (because we can't, or couldn't, access separate submodule repos
otherwise). But eventually we need to access objects from the right
repo.

Because the same commit could sometimes be present in both 'submodule'
and 'sub', if there is a bug looking up objects in the wrong repo,
sometimes it will go unnoticed because it finds the needed object in the
wrong repo anyway.

Fix this by changing commit time after every commit. This makes all
commits unique. Of course there are still identical blobs in different
repos, but because we often lookup commit first, then tree and blob,
unique commits are already quite safe.

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t7814-grep-recurse-submodules.sh

@@ -14,12 +14,14 @@ test_expect_success 'setup directory structure and submodule' '
 	echo "(3|4)" >b/b &&
 	git add a b &&
 	git commit -m "add a and b" &&
+	test_tick &&
 	git init submodule &&
 	echo "(1|2)d(3|4)" >submodule/a &&
 	git -C submodule add a &&
 	git -C submodule commit -m "add a" &&
 	git submodule add ./submodule &&
-	git commit -m "added submodule"
+	git commit -m "added submodule" &&
+	test_tick
 '
 
 test_expect_success 'grep correctly finds patterns in a submodule' '
@@ -65,11 +67,14 @@ test_expect_success 'grep and nested submodules' '
 	echo "(1|2)d(3|4)" >submodule/sub/a &&
 	git -C submodule/sub add a &&
 	git -C submodule/sub commit -m "add a" &&
+	test_tick &&
 	git -C submodule submodule add ./sub &&
 	git -C submodule add sub &&
 	git -C submodule commit -m "added sub" &&
+	test_tick &&
 	git add submodule &&
 	git commit -m "updated submodule" &&
+	test_tick &&
 
 	cat >expect <<-\EOF &&
 	a:(1|2)d(3|4)
@@ -179,15 +184,18 @@ test_expect_success !MINGW 'grep recurse submodule colon in name' '
 	echo "(1|2)d(3|4)" >"parent/fi:le" &&
 	git -C parent add "fi:le" &&
 	git -C parent commit -m "add fi:le" &&
+	test_tick &&
 
 	git init "su:b" &&
 	test_when_finished "rm -rf su:b" &&
 	echo "(1|2)d(3|4)" >"su:b/fi:le" &&
 	git -C "su:b" add "fi:le" &&
 	git -C "su:b" commit -m "add fi:le" &&
+	test_tick &&
 
 	git -C parent submodule add "../su:b" "su:b" &&
 	git -C parent commit -m "add submodule" &&
+	test_tick &&
 
 	cat >expect <<-\EOF &&
 	fi:le:(1|2)d(3|4)
@@ -210,15 +218,18 @@ test_expect_success 'grep history with moved submoules' '
 	echo "(1|2)d(3|4)" >parent/file &&
 	git -C parent add file &&
 	git -C parent commit -m "add file" &&
+	test_tick &&
 
 	git init sub &&
 	test_when_finished "rm -rf sub" &&
 	echo "(1|2)d(3|4)" >sub/file &&
 	git -C sub add file &&
 	git -C sub commit -m "add file" &&
+	test_tick &&
 
 	git -C parent submodule add ../sub dir/sub &&
 	git -C parent commit -m "add submodule" &&
+	test_tick &&
 
 	cat >expect <<-\EOF &&
 	dir/sub/file:(1|2)d(3|4)
@@ -229,6 +240,7 @@ test_expect_success 'grep history with moved submoules' '
 
 	git -C parent mv dir/sub sub-moved &&
 	git -C parent commit -m "moved submodule" &&
+	test_tick &&
 
 	cat >expect <<-\EOF &&
 	file:(1|2)d(3|4)
@@ -251,6 +263,7 @@ test_expect_success 'grep using relative path' '
 	echo "(1|2)d(3|4)" >sub/file &&
 	git -C sub add file &&
 	git -C sub commit -m "add file" &&
+	test_tick &&
 
 	git init parent &&
 	echo "(1|2)d(3|4)" >parent/file &&
@@ -260,6 +273,7 @@ test_expect_success 'grep using relative path' '
 	git -C parent add src/file2 &&
 	git -C parent submodule add ../sub &&
 	git -C parent commit -m "add files and submodule" &&
+	test_tick &&
 
 	# From top works
 	cat >expect <<-\EOF &&
@@ -293,6 +307,7 @@ test_expect_success 'grep from a subdir' '
 	echo "(1|2)d(3|4)" >sub/file &&
 	git -C sub add file &&
 	git -C sub commit -m "add file" &&
+	test_tick &&
 
 	git init parent &&
 	mkdir parent/src &&
@@ -301,6 +316,7 @@ test_expect_success 'grep from a subdir' '
 	git -C parent submodule add ../sub src/sub &&
 	git -C parent submodule add ../sub sub &&
 	git -C parent commit -m "add files and submodules" &&
+	test_tick &&
 
 	# Verify grep from root works
 	cat >expect <<-\EOF &&