Refined commit-check

My recent small change included Copyright-paperwork-exempt: Yes in the commit message but the script contrib/commit-check did not handle this correctly. So, I submit a small set of changes.

1. Detect Copyright-paperwork-exempt: Yes correctly
   This script used git log to find a line that begins with the string above but git log automatically indents the commit message and the script will not work correctly. I changed this to use git cat-file -p HEAD (pretty-print HEAD commit) to prevent auto-indent.
2. [REMOVED]

Note

1. I recently signed FSF Copyright Assignment for GNU Wget but not signed by FSF's Deputy Director yet. I think it will complete soon.
2. Should grep "^Copyright-paperwork-exempt: Yes" be grep '^Copyright-paperwork-exempt: Yes$'? Or do we need any relaxed regex pattern?

Approver's checklist:

• The author has submitted the FSF Copyright Assignment and is listed in AUTHORS
• There is a test suite reasonably covering new functionality or modifications
• Function naming, parameters, return values, types, etc., are consistent with existing code
• This feature/change has adequate documentation added (if appropriate)
• No obvious mistakes / misspelling in the code