t/t9300-fast-import.sh: prepare for `gc --cruft` by default

t/t9300-fast-import.sh: prepare for `gc --cruft` by default

In a similar fashion as previous commits, adjust the fast-import tests
to prepare for "git gc" generating a cruft pack by default.

This adjustment is slightly different, however. Instead of relying on us
writing out the objects loose, and then calling `git prune` to remove
them, t9300 needs to be prepared to drop objects that would be moved
into cruft packs.

To do this, we can combine the `git gc` invocation with `git prune` into
one `git gc --prune`, which handles pruning both loose objects, and
objects that would otherwise be written to a cruft pack.

Likely this pattern of "git gc && git prune" started all the way back in
03db4525d3 (Support gitlinks in fast-import., 2008-07-19), which
happened after deprecating `git gc --prune` in 9e7d501990 (builtin-gc.c:
deprecate --prune, it now really has no effect, 2008-05-09).

After `--prune` was un-deprecated in 58e9d9d472 (gc: make --prune useful
again by accepting an optional parameter, 2009-02-14), this script got a
handful of new "git gc && git prune" instances via via 4cedb78cb5
(fast-import: add input format tests, 2011-08-11). These could have been
`git gc --prune`, but weren't (likely taking after 03db4525d3).

Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>