fix crash when generating a unified diff with a small ending equal-chunk

Signed-off-by: Mechiel Lukkien <[email protected]>

Author: mjl-

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
 	}
 }