golden version; handle git log output; update README

README.md

 ## About
 
-Diff viewer.
+Diff viewer, side-by-side, auto pager with `less`.
 
+## Install
+
+Just download the `src/cdiff.py` and save to whatever directory which in your
+`$PATH`, for example, `$HOME/bin` is in my `$PATH`, so I save the script there
+and name as `cdiff`.
+
+    curl -ksS https://raw.github.com/ymattw/cdiff/master/src/cdiff.py > ~/bin/cdiff
+    
 ## Usage
+    
+Read diff from svn, use option `-s` for side-by-side view, use option `-w` to
+use text width other than default `80`.  You don't need `less`, it's automatic:
+
+    svn diff | cdiff
+    svn diff | cdiff -s
+    svn diff | cdiff -s -w 90
+    
+Read diff from git:
+
+    git diff | cdiff -s
+    git log -p -2 | cdiff -s
+    git show <commit> | cdiff -s
+
+View a diff (patch) file:
+
+    cdiff foo.patch
+    cdiff foo.patch -s
+    cdiff foo.patch -s -w 90
+
+Redirect output to another patch file is safe:
 
-TODO
+    svn diff | cdiff -s > my.patch

src/cdiff.py

         hunk = None
 
         while stream:
-            if Udiff.is_header(stream[0]):
+            # 'common' line occurs before 'old_path' is considered as header
+            # too, this happens with `git log -p` and `git show <commit>`
+            #
+            if Udiff.is_header(stream[0]) or \
+                    (Udiff.is_common(stream[0]) and old_path is None):
                 if headers and old_path:
                     # Encounter a new header
                     assert new_path is not None