add test for crashing diff

this reuses an existing patch, setting context to 6 triggers the
bug, becuase of a 5-line trailing equals chunk.

Signed-off-by: Mechiel Lukkien <mechiel@ueber.net>

Author: mjl-

plumbing/format/diff/unified_encoder_test.go

@@ -475,6 +475,43 @@ index ab5eed5d4a2c33aeef67e0188ee79bed666bde6f..0adddcde4fd38042c354518351820eb0
  V
  W
 `,
+}, {
+	patch:   oneChunkPatch,
+	desc:    "modified deleting lines file with context to 6",
+	context: 6,
+	diff: `diff --git a/onechunk.txt b/onechunk.txt
+index ab5eed5d4a2c33aeef67e0188ee79bed666bde6f..0adddcde4fd38042c354518351820eb06c417c82 100644
+--- a/onechunk.txt
++++ b/onechunk.txt
+@@ -1,27 +1,23 @@
+-A
+ B
+ C
+ D
+ E
+ F
+ G
+-H
+ I
+ J
+ K
+ L
+ M
+ N
+-Ñ
+ O
+ P
+ Q
+ R
+ S
+ T
+-U
+ V
+ W
+ X
+ Y
+ Z
+`,
 }, {
 	patch: oneChunkPatch,