plumbing: diff, fix crash when a small ending equal-chunk

plumbing/format/diff/unified_encoder.go

@@ -262,11 +262,15 @@ func (c *hunksGenerator) processEqualsLines(ls []string, i int) {
 		c.current.AddOp(Equal, c.afterContext...)
 		c.afterContext = nil
 	} else {
-		c.current.AddOp(Equal, c.afterContext[:c.ctxLines]...)
+		ctxLines := c.ctxLines
+		if ctxLines > len(c.afterContext) {
+			ctxLines = len(c.afterContext)
+		}
+		c.current.AddOp(Equal, c.afterContext[:ctxLines]...)
 		c.hunks = append(c.hunks, c.current)
 
 		c.current = nil
-		c.beforeContext = c.afterContext[c.ctxLines:]
+		c.beforeContext = c.afterContext[ctxLines:]
 		c.afterContext = nil
 	}
 }

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,