checkkconfigsymbols.py: make it Git aware

The script now supports to check a specified commit or a specified range
of commits (i.e., commit1..commit2).  Developers and maintainers are
encouraged to use this functionality before sending or merging patches
to avoid potential bugs and to keep the code, documentation, etc. clean.

This patch adds the following options to the script:

 -c COMMIT, --commit=COMMIT
                  Check if the specified commit (hash) introduces
                  undefined Kconfig symbols.

 -d DIFF, --diff=DIFF
                  Diff undefined symbols between two commits.  The input
                  format bases on Git log's 'commmit1..commit2'.

  --force         Reset current Git tree even when it's dirty.

Note that the first two options require to 'git reset --hard' the user's
Git tree.  This hard reset is necessary to keep the script fast, but it
can lead to the loss of uncommitted data.  Hence, the script aborts in
case it is executed in a dirty tree.  It won't abort if '--force' is
passed.

If neither -c nor -d is specified, the script defaults to check the
entire local tree (i.e., the previous behavior).

Signed-off-by: Valentin Rothberg <valentinrothberg@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>