Fix "git checkout -b foo ':/substring'"

Because ':/substring' extended SHA1 expression cannot take
postfix modifiers such as ^{tree} and ^{commit}, we would need
to do it in multiple steps.  With the patch, you can start a new
branch from a randomly-picked commit whose message has the named
string in it.

Signed-off-by: Junio C Hamano <gitster@pobox.com>