When an option could be an ambiguous abbreviation of two options, the code
used to error out.  Even if an exact match would have occured later.

Test and original patch by Pierre Habouzit.
Signed-off-by: NJohannes Schindelin <Johannes.Schindelin@gmx.de>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>