[git-cl] Preserve line breaks when pretty printing CL description.

Also set the line wrap limit to 72 + indent from 70 columns total. The
latter was limiting the CL description to 68 columns total before
wrapping kicked in.

BUG=none

Change-Id: I93c984c7b121d4bb042d0dc81a662352f77df4d1
Reviewed-on: https://chromium-review.googlesource.com/420243Reviewed-by: Dirk Pranke <dpranke@chromium.org>
Commit-Queue: Dirk Pranke <dpranke@chromium.org>

git_cl.py

@@ -1421,9 +1421,11 @@ class Changelist(object):
         self.description = self._codereview_impl.FetchDescription()
       self.has_description = True
     if pretty:
-      wrapper = textwrap.TextWrapper()
+      # Set width to 72 columns + 2 space indent.
+      wrapper = textwrap.TextWrapper(width=74, replace_whitespace=True)
       wrapper.initial_indent = wrapper.subsequent_indent = '  '
-      return wrapper.fill(self.description)
+      lines = self.description.splitlines()
+      return '\n'.join([wrapper.fill(line) for line in lines])
     return self.description
 
   def GetPatchset(self):