Improve memory efficiency for test data preparation

Data preparation for huge_patch test could be very slow because strings
are immutable, each concatenation creates a new string and discards
the old ones.

New approach of data preparation is to concatenate large string from
smaller parts with ''.join() method. This method reduces memory usage
and enhances performance, because it minimizes the number of new string
objects created.

tests/test_patch.py

@@ -1426,7 +1426,7 @@ def test_svn_mixed_line_ends(self):
         self.assertEqual(results[0].header, expected_header)
 
     def test_huge_patch(self):
-        text = """diff --git a/huge.file b/huge.file
+        text_parts = ["""diff --git a/huge.file b/huge.file
 index 0000000..1111111 100644
 --- a/huge.file
 +++ a/huge.file
@@ -1438,9 +1438,9 @@ def test_huge_patch(self):
 -44444444
 +55555555
 +66666666
-"""
-        for n in range(0, 1000000):
-            text += "+" + hex(n) + "\n"
+"""]
+        text_parts.extend("+" + hex(n) + "\n" for n in range(0, 1000000))
+        text = ''.join(text_parts)
         start_time = time.time()
         result = list(wtp.patch.parse_patch(text))
         self.assertEqual(1, len(result))