Until now, we treated "--pretty=" or "--format=" as "give me
the default format". This was not planned nor documented,
but only what happened to work due to our parsing of
"--pretty" (which should give the default format).

Let's instead let these be an actual empty userformat.
Otherwise one must write out the annoyingly long
"--pretty=tformat:" to get the same behavior.
Signed-off-by: NJeff King <peff@peff.net>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>