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
Merge "docs should indicate proper git commit limit"