git-p4: auto-size the block

git-p4: auto-size the block

git-p4 originally would fetch changes in one query. On large repos this
could fail because of the limits that Perforce imposes on the number of
items returned and the number of queries in the database.

To fix this, git-p4 learned to query changes in blocks of 512 changes,
However, this can be very slow - if you have a few million changes,
with each chunk taking about a second, it can be an hour or so.

Although it's possible to tune this value manually with the
"--changes-block-size" option, it's far from obvious to ordinary users
that this is what needs doing.

This change alters the block size dynamically by looking for the
specific error messages returned from the Perforce server, and reducing
the block size if the error is seen, either to the limit reported by the
server, or to half the current block size.

That means we can start out with a very large block size, and then let
it automatically drop down to a value that works without error, while
still failing correctly if some other error occurs.

Signed-off-by: Luke Diamand <luke@diamand.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>