checkkconfigsymbols.py: Forbid passing 'HEAD' to --commit

As opposed to the --diff option, --commit can get ref names instead of
commit hashes.

When using the --commit option, the script resets the working directory
to the commit before the given ref, by adding '~' to the end of the ref.

However, the 'HEAD' ref is relative, and so when the working directory
is reset to 'HEAD~', 'HEAD' points to what was 'HEAD~'. Then when the
script resets to 'HEAD' it actually stays in the same commit. In this
case, the script won't report any cases because there is no diff between
the cases of the two refs.

Prevent the user from using HEAD refs.

A better solution might be to resolve the refs before doing the
reset, but for now just disallow such refs.

Signed-off-by: Ariel Marcovitch <arielmarcovitch@gmail.com>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>