checkkconfigsymbols.py: find relevant commits

Add option -f/--find to find relevant commits when using the --diff
option.  --find is useful in case a user wants to check commits that
potentially cause a Kconfig symbol to be missing.  This is done via 'git
log -G $SYMBOL' (i.e., to get a list of commits that change $SYMBOL).
The relevant commits are printed below the "SYMBOL\tFILES" line,
followed by an empty line to increase readability.

Signed-off-by: Valentin Rothberg <valentinrothberg@gmail.com>
Acked-by: Stefan Hengelein <stefan.hengelein@fau.de>
Acked-by: Andreas Ruprecht <andreas.ruprecht@fau.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>