format-patch: inform user that patch-id generation is unstable

I tried out 'git format-patch --base' with a set of commits that
modifies more than one file. It turns out that the way this command is
implemented it actually uses the unstable version of patch-id instead of
the stable version that's documented. When I tried to modify the
existing test to use 'git patch-id --stable' vs. 'git patch-id
--unstable' I found that it didn't matter, the test still passed.

Let's expand on the test here so it is a little more complicated and
then use that to show that the patch-id generation is actually unstable
vs. stable. Update the documentation as well.

Cc: Xiaolong Ye <xiaolong.ye@intel.com>
Signed-off-by: Stephen Boyd <sboyd@kernel.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-format-patch.txt

@@ -583,7 +583,7 @@ of 'base commit' in topological order before the patches can be applied.
 The 'base commit' is shown as "base-commit: " followed by the 40-hex of
 the commit object name.  A 'prerequisite patch' is shown as
 "prerequisite-patch-id: " followed by the 40-hex 'patch id', which can
-be obtained by passing the patch through the `git patch-id --stable`
+be obtained by passing the patch through the `git patch-id --unstable`
 command.
 
 Imagine that on top of the public commit P, you applied well-known

t/t4014-format-patch.sh

@@ -36,8 +36,27 @@ test_expect_success setup '
 	git checkout master &&
 	git diff-tree -p C2 | git apply --index &&
 	test_tick &&
-	git commit -m "Master accepts moral equivalent of #2"
+	git commit -m "Master accepts moral equivalent of #2" &&
 
+	git checkout side &&
+	git checkout -b patchid &&
+	for i in 5 6 1 2 3 A 4 B C 7 8 9 10 D E F; do echo "$i"; done >file2 &&
+	for i in 1 2 3 A 4 B C 7 8 9 10 D E F 5 6; do echo "$i"; done >file3 &&
+	for i in 8 9 10; do echo "$i"; done >file &&
+	git add file file2 file3 &&
+	test_tick &&
+	git commit -m "patchid 1" &&
+	for i in 4 A B 7 8 9 10; do echo "$i"; done >file2 &&
+	for i in 8 9 10 5 6; do echo "$i"; done >file3 &&
+	git add file2 file3 &&
+	test_tick &&
+	git commit -m "patchid 2" &&
+	for i in 10 5 6; do echo "$i"; done >file &&
+	git add file &&
+	test_tick &&
+	git commit -m "patchid 3" &&
+
+	git checkout master
 '
 
 test_expect_success "format-patch --ignore-if-in-upstream" '
@@ -1559,16 +1578,23 @@ test_expect_success 'format-patch -o overrides format.outputDirectory' '
 '
 
 test_expect_success 'format-patch --base' '
-	git checkout side &&
+	git checkout patchid &&
 	git format-patch --stdout --base=HEAD~3 -1 | tail -n 7 >actual1 &&
 	git format-patch --stdout --base=HEAD~3 HEAD~.. | tail -n 7 >actual2 &&
 	echo >expected &&
 	echo "base-commit: $(git rev-parse HEAD~3)" >>expected &&
-	echo "prerequisite-patch-id: $(git show --patch HEAD~2 | git patch-id --stable | awk "{print \$1}")" >>expected &&
-	echo "prerequisite-patch-id: $(git show --patch HEAD~1 | git patch-id --stable | awk "{print \$1}")" >>expected &&
+	echo "prerequisite-patch-id: $(git show --patch HEAD~2 | git patch-id --unstable | awk "{print \$1}")" >>expected &&
+	echo "prerequisite-patch-id: $(git show --patch HEAD~1 | git patch-id --unstable | awk "{print \$1}")" >>expected &&
 	signature >> expected &&
 	test_cmp expected actual1 &&
-	test_cmp expected actual2
+	test_cmp expected actual2 &&
+	echo >fail &&
+	echo "base-commit: $(git rev-parse HEAD~3)" >>fail &&
+	echo "prerequisite-patch-id: $(git show --patch HEAD~2 | git patch-id --stable | awk "{print \$1}")" >>fail &&
+	echo "prerequisite-patch-id: $(git show --patch HEAD~1 | git patch-id --stable | awk "{print \$1}")" >>fail &&
+	signature >> fail &&
+	! test_cmp fail actual1 &&
+	! test_cmp fail actual2
 '
 
 test_expect_success 'format-patch --base errors out when base commit is in revision list' '