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>