don't use default revision if a rev was specified

If a revision is specified, it happens not to have any commits, don't
use the default revision.  By doing so, surprising and undesired
behavior can happen, such as showing the reflog for HEAD when a branch
was specified.

[jc: squashed a test from René]

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