shortlog: default to HEAD when the standard input is a tty

shortlog: default to HEAD when the standard input is a tty

Instead of warning the user that it is expecting git log output from
the standard input (and waiting for the user to type the log from
the keyboard, which is a silly thing to do), default to traverse from
HEAD when there is no rev parameter given and the standard input is
a tty.

This factors out a useful helper "add_head()" from builtin-diff.c to a
more appropriate place revision.c while renaming it to more descriptive
name add_head_to_pending(), as that is what the function is about.

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