Don't lint hard tabs in comments

The default git commit message includes a 'Changes to be committed'
section which has hard tabs (ghasp) in comments.

We shouldn't warn the user about these tabs. This runs
`user_commit_message`, which only includes non-comment lines (in
addition to stripping git diff output when running `git commit
--verbose`)

Change-Id: Ib9163a55508c448c745b5aa5bd54508511ea7b03
Reviewed-on: https://gerrit.causes.com/23511
Tested-by: jenkins <jenkins@causes.com>
Reviewed-by: Lann Martin <lann@causes.com>
Reviewed-by: Greg Hurrell <greg@causes.com>