revert: Don't check lone argument in get_encoding

revert: Don't check lone argument in get_encoding

The only place get_encoding uses the global "commit" variable is when
writing an error message explaining that its lone argument was NULL.
Since the function's only caller ensures that a NULL argument isn't
passed, we can remove this check with two beneficial consequences:

1. Since the function doesn't use the global "commit" variable any
   more, it won't need to change when we eliminate the global variable
   later in the series.
2. Translators no longer need to localize an error message that will
   never be shown.

Suggested-by: Junio C Hamano <gitster@pobox.com>
Mentored-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Ramkumar Ramachandra <artagnon@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>