committers-guide: increase subject length limit to 67 characters

[emaste]

Summary

I originally suggested a 63-character limit "so that git log --oneline
avoids wrapping".  Short hashes in the tree are typically 12 characters.
With a space between the hash and the subject should result in a limit
of 67 characters.  I think I arrived at 63 by subtracting git's typical
4-space indentation, but that's not applicable to `git log --oneline`.

[glebius]

btw, I got this script. Named wrap74, it would wrap lines to 74 chars. And you can have several symlinks/hardlinks for it, with different names, e.g. wrap79, wrap64.

#!/usr/local/bin/python

import fileinput
import textwrap
import os
import re

len = int(re.search('[0-9]+$', os.path.basename(__file__)).group())

all = ''
for line in fileinput.input():
        if (line == '\n'):
                print(textwrap.fill(all, len))
                print()
                all = ''
        else:
                all += line

print(textwrap.fill(all, len))