Teach rev-list an option to read revs from the standard input.

Teach rev-list an option to read revs from the standard input.

When --stdin option is given, in addition to the <rev>s listed
on the command line, the command can read one rev parameter per
line from the standard input.  The list of revs ends at the
first empty line or EOF.

Note that you still have to give all the flags from the command
line; only rev arguments (including A..B, A...B, and A^@ notations)
can be give from the standard input.

Signed-off-by: Junio C Hamano <redacted>