projtool.pl: add gc subcommand
The gc subcommand provides a convenient front-end to running the
gc.sh script on a project.
Ultimately it just runs the gc.sh script on the project.
However, it provides semantics that are more similar to those
of the `git gc` command:
* show_progress=1 is the default unless --quiet is used
* at least a mini gc will happen unless --auto is used
* a full gc can be forced with --force (or the stealth
alias --aggressive)
* with --redelta (or the stealth alias --no-reuse-delta)
deltas will be fully recomputed
Signed-off-by: Kyle J. McKay <mackyle@gmail.com>