t4203: do not let "git shortlog" DWIM based on tty

t4203: do not let "git shortlog" DWIM based on tty

The "shortlog" command defaults to HEAD only when its standard input is
connected to a terminal; otherwise it acts in the traditional "filter"
mode to read and summarize the "git log" output.

Two new tests added to t4203 assumed that the command always default to
HEAD, but when the standard input is closed (or connected to /dev/null),
it output empty, which is a summary of its empty input, causing the test
to break.

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