Do not misidentify "git merge foo HEAD" as an old-style invocation

Do not misidentify "git merge foo HEAD" as an old-style invocation

This was misinterpreted as an ancient style "git merge <message> HEAD
<commit> <commit>..." that merges one (or more) <commit> into the current
branch and record the resulting commit with the given message.  Then a
later sanity check found that there is no <commit> specified and gave
a usage message.

Tested-by: Nanako Shiraishi <nanako3@lavabit.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>