It is possible to break your repository config by creating an invalid key.  The
config parser in turn chokes on it:

  $ git init
  Initialized empty Git repository in /tmp/gittest/.git/
  $ git config .foo false
  $ git config core.bare
  fatal: bad config file line 6 in .git/config

This patch makes git-config reject keys which start or end with a dot and adds
tests for these cases.
Signed-off-by: NLibor Pechacek <lpechacek@suse.cz>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>