t/t5333-pseudo-merge-bitmaps.sh: demonstrate empty pseudo-merge groups

Demonstrate that it is possible to generate empty pseudo-merge commits
in certain cases.

In the below instance, we generate one non-empty pseudo-merge
(containing commit "base"), and one empty pseudo-merge group
(corresponding to the unstable commits within that group).

(In my testing, the pseudo-merge machinery seems to handle empty groups
just fine, but generating them is pointless as they carry no
information.)

This commit (introducing a deliberate "test_expect_failure") is split
out from the actual fix (which will appear in the following commit) to
demonstrate that the failure is correctly induced.

Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t5333-pseudo-merge-bitmaps.sh

@@ -390,4 +390,24 @@ test_expect_success 'pseudo-merge reuse' '
 	)
 '
 
+test_expect_failure 'empty pseudo-merge group' '
+	git init pseudo-merge-empty-group &&
+	(
+		cd pseudo-merge-empty-group &&
+
+		# Ensure that a pseudo-merge group with no unstable
+		# commits does not generate an empty pseudo-merge
+		# bitmap.
+		git config bitmapPseudoMerge.empty.pattern refs/ &&
+
+		test_commit base &&
+		git repack -adb &&
+
+		test-tool bitmap dump-pseudo-merges >merges &&
+		test_line_count = 1 merges &&
+
+		test 0 -eq "$(grep -c commits=0 <merges)"
+	)
+'
+
 test_done