http-fetch: make `-a` standard behaviour

http-fetch: make `-a` standard behaviour

This is a follow-up to a6c786fce8 (Mark http-fetch without -a as
deprecated, 2011-08-23). For more than six years, we have been warning
when `-a` is not provided, and the documentation has been saying that
`-a` will become the default.

It is a bit unclear what "default" means here. There is no such thing as
`http-fetch --no-a`. But according to my searches, no-one has been
asking on the mailing list how they should silence the warning and
prepare for overriding the flipped default. So let's assume that
everybody is happy with `-a`. They should be, since not using it may
break the repo in such a way that Git itself is unable to fix it.

Always behave as if `-a` was given. Since `-a` implies `-c` (get commit
objects) and `-t` (get trees), all three options are now unnecessary.
Document all of these as historical artefacts that have no effect.

Leave no-op code for handling these options in http-fetch.c. The
options-handling is currently rather loose. If someone tightens it, we
will not want these ignored options to accidentally turn into hard
errors.

Since `-a` was the only safe and sane usage and we have been pushing
people towards it for a long time, refrain from warning when it is used
"unnecessarily" now. Similarly, do not add anything scary-looking to the
man-page about how it will be removed in the future. We can always do so
later. (It is not like we are in desperate need of freeing up
one-letter arguments.)

Signed-off-by: Martin Ågren <martin.agren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>