projtool.pl: accept gc --aggressive for --force --redelta

projtool.pl: accept gc --aggressive for --force --redelta

The standard `git gc` command accepts no options, `--auto` and
`--aggressive` to activate various modes.  By adding support for
`--aggressive` it becomes a simple matter to translate the normal
`git gc` command into the equivalent `projtool.pl gc` command without
needing to exercise the gray matter.

The equivalent of `git gc --aggressive` as far as `projtool.pl gc`
is concerned is just `projtool.pl gc --force --redelta` which is
already supported.

Make `--aggressive` activate the `--force` and `--redelta` options.

Since a plain `projtool.pl gc` already roughly corresponds to a
plain `git gc` and a `projtool.pl gc --auto` already roughly
corresponds to `git gc --auto` this change completes the trifecta.

Signed-off-by: Kyle J. McKay <mackyle@gmail.com>