docs should indicate proper git commit limit

The nova gate should recommend 50 characters or less for a git commit
but actually enforce 72 characters.

This patch changes the hacking.rst docs to indicate the actual limit
is 72 characters rather then 50 characters.

Change-Id: I47f1f1f1007f5744bf1fef419df7e033803b4a53
Fixes: Bug #1144840

HACKING.rst

@@ -271,8 +271,8 @@ Commit Messages
 Using a common format for commit messages will help keep our git history
 readable. Follow these guidelines:
 
-  First, provide a brief summary (it is recommended to keep the commit title
-  under 50 chars).
+  First, provide a brief summary of 50 characters or less.  Summaries
+  of greater then 72 characters will be rejected by the gate.
 
   The first line of the commit message should provide an accurate
   description of the change, not just a reference to a bug or

tools/hacking.py

@@ -596,7 +596,7 @@ def once_git_check_commit_title():
     nova HACKING recommends not referencing a bug or blueprint in first line,
     it should provide an accurate description of the change
     N801
-    N802 Title limited to 50 chars
+    N802 Title limited to 72 chars
     """
     #Get title of most recent commit
 
@@ -621,6 +621,8 @@ def once_git_check_commit_title():
                "description of the change, not just a reference to a bug "
                "or blueprint" % title.strip())
         error = True
+    # HACKING.rst recommends commit titles 50 chars or less, but enforces
+    # a 72 character limit
     if len(title.decode('utf-8')) > 72:
         print ("N802: git commit title ('%s') should be under 50 chars"
                 % title.strip())