Handle os.path.commonprefix exceptions (#124)

fix multidrive win chop

On Windows, when using different drives between source and report, `report.chop` raises an exception "ValueError("path is on mount %r, start on mount %r" % (path_drive, start_drive))"
(see https://github.com/python/cpython/blob/master/Lib/ntpath.py#L703)

libscanbuild/report.py

@@ -492,8 +492,13 @@ def safe_readlines(filename):
 def chop(prefix, filename):
     # type: (str, str) -> str
     """ Create 'filename' from '/prefix/filename' """
-
-    return filename if not prefix else os.path.relpath(filename, prefix)
+    result = filename
+    if prefix:
+        try:
+            result = os.path.relpath(filename, prefix)
+        except ValueError:
+            pass
+    return result
 
 
 def escape(text):

tests/unit/test_report.py

@@ -116,6 +116,8 @@ class ReportMethodTest(unittest.TestCase):
         self.assertEqual('lib\\file',
                          sut.chop('c:\\prefix\\', 'c:\\prefix\\lib\\file'))
         self.assertEqual('c:\\prefix\\file', sut.chop('', 'c:\\prefix\\file'))
+        self.assertEqual('c:\\prefix\\file',
+                         sut.chop('e:\\prefix', 'c:\\prefix\\file'))
 
     @unittest.skipIf(not IS_WINDOWS, 'windows has different path patterns')
     def test_chop_when_cwd_on_windows(self):