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>