Disallow empty section and variable names

Disallow empty section and variable names

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: Libor Pechacek <redacted>
Signed-off-by: Junio C Hamano <redacted>