[lit] Fix problem in how Python versions open files with different encodings

Reapply D43165 which was reverted because of different versions of python failing. 
The one line fix for the different python versions was commited at the same time
that D43165 was reverted. If this change is giving you issues then get in touch
with your python version and we will fix it. 

llvm-svn: 329022

llvm/utils/lit/lit/TestRunner.py

@@ -388,7 +388,47 @@ def executeBuiltinDiff(cmd, cmd_shenv):
     def compareTwoFiles(filepaths):
         filelines = []
         for file in filepaths:
-            with io.open(file, 'r') as f:
+            try:
+                with open(file, 'r') as f:
+                    filelines.append(f.readlines())
+            except UnicodeDecodeError:
+                try:
+                    with open(file, 'r', encoding="utf-8") as f:
+                        filelines.append(f.readlines())
+                    encoding = "utf-8"
+                except:
+                    compare_bytes = True
+
+        if compare_bytes:
+            return compareTwoBinaryFiles(filepaths)
+        else:
+            return compareTwoTextFiles(filepaths, encoding)
+
+    def compareTwoBinaryFiles(filepaths):
+        filelines = []
+        for file in filepaths:
+            with open(file, 'rb') as f:
+                filelines.append(f.readlines())
+
+        exitCode = 0 
+        if hasattr(difflib, 'diff_bytes'):
+            # python 3.5 or newer
+            diffs = difflib.diff_bytes(difflib.unified_diff, filelines[0], filelines[1], filepaths[0].encode(), filepaths[1].encode())
+            diffs = [diff.decode() for diff in diffs]
+        else:
+            # python 2.7
+            func = difflib.unified_diff if unified_diff else difflib.context_diff
+            diffs = func(filelines[0], filelines[1], filepaths[0], filepaths[1])
+
+        for diff in diffs:
+            stdout.write(diff)
+            exitCode = 1
+        return exitCode
+
+    def compareTwoTextFiles(filepaths, encoding):
+        filelines = []
+        for file in filepaths:
+            with io.open(file, 'r', encoding=encoding) as f:
                 filelines.append(f.readlines())
 
         exitCode = 0