clone: use --progress to force progress reporting

clone: use --progress to force progress reporting

Follow the argument convention of git-pack-objects, such that a
separate option (--preogress) is used to force progress reporting
instead of -v/--verbose.

-v/--verbose now does not force progress reporting. Make git-clone.txt
say so.

This should cover all the bases in 21188b1 (Implement git clone -v),
which implemented the option to force progress reporting.

Signed-off-by: Tay Ray Chuan <rctay89@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>