Fix auto-abbreviation of author names

When all author names are less than 11 characters long all author names
would automatically be abbreviated. For example running this command in
the Git repository would show "Jeff King" as "JKing":

    > tig -S--porcelain Documentation/git-status.txt

Fix this by passing in the configured author column width instead of the
calculated column width.
Improve display of commas and spaces in the help view
Rewrite index diffing to use git-status

Use the porcelain format of git-status to only execute one command
instead of three when querying the index for changes. The API has been
kept more rich than necessary so it can be used to improve the code
watching for index changes.