patch: turned strings with backslashes into raw strings

In Python, the backslash in an unrecognized escape sequence is left
behind, which makes '\.' the same as r'\.'. Relying on this feature is
quite brittle, IMHO.

Removed unnecessary string concatenation as well.

mercurial/patch.py

@@ -52,9 +52,9 b' def extract(ui, fileobj):'
 
     # attempt to detect the start of a patch
     # (this heuristic is borrowed from quilt)
-    diffre = re.compile(r'^(?:Index:[ \t]|diff[ \t]|RCS file: |' +
-                        'retrieving revision [0-9]+(\.[0-9]+)*$|' +
-                        '(---|\*\*\*)[ \t])', re.MULTILINE)
+    diffre = re.compile(r'^(?:Index:[ \t]|diff[ \t]|RCS file: |'
+                        r'retrieving revision [0-9]+(\.[0-9]+)*$|'
+                        r'(---|\*\*\*)[ \t])', re.MULTILINE)
 
     fd, tmpname = tempfile.mkstemp(prefix='hg-patch-')
     tmpfp = os.fdopen(fd, 'w')