Setting sensible defaults for some CLI arguments

[bbhtt]

• --state-dir - Default to a global state dir like ~/.cache/flatpak-builder to enable sharing of downloaded artifacts and ccache artifacts across multiple manifests. This should speed up downloads and builds for someone building multiple things. The requirement is that the checkout app dir and state dir must stay on the same filesystem. This will also prevent people git add-ing .flatpak-builder.

• --ccache - Default it to on, every SDK ships ccache nowadays and if someone doesn't they should. See also make --ccache on by default #582

• --subject - Set to SHA of head if inside a git directory. Already done by flatpak-github-actions and Flathub since ages.

• --body - Set to something sensible like a hash of the manifest from state directory.

• --force-clean - Enable by default. Every new build should never reuse the checked out app dir and it currently errors anyway.

• --override-source-date-epoch - Set to a fixed date to enable reproducible builds by default. Manifest last modification timestamp is not reproducibile (see No option to get a deterministic build timestamp flathub/flathub#2617) and a fixed date has been the standard for reproducible builds since a long time.

• --compose-url-policy - Default to 'full' to get back the appstream-glib behaviour which was lost during libappstream transition in 1.3.x. We will require and link against libappstream >=1.1.2 anyways.

• --user - Default to at least installing on the Flatpak user installation instead of system. Building and installing as user makes more sense and the system code path has been forever broken due to requiring auth etc flatpak-builder --install leads to "error: Flatpak system operation ConfigureRemote not allowed for user" #290

For each of these options, the plan is to

• change the default
• mark the old argument as deprecated (probably in the code and docs only and not any user facing warning)
• hide it from help
• introduce a negative --disable-foo where it makes sense
• update the documentation

[Erick555]

--force-clean - Enable by default. Every new build should never reuse the checked out app dir and it currently errors anyway.

Wouldn't it risk wiping content that wasn't supposed to be removed? I remember people wiping their entire home in the past but at least they explicitly passed this.

--user - Default to at least installing on the Flatpak user installation instead of system.

That would make it inconsistent with flatpak default behavior.

[bbhtt]

Accidental deletion should be less of a problem since #668. It's not 100% but unless people use weird symlinks or weird invocations like below it shouldn't happen for most of the day to day uses.

cd /tmp
flatpak-builder --force-clean ~/ $manifest

It's not set in stone as I am thinking about making it create a new builddir if one is occupied too, to solve the problem.

That would make it inconsistent with flatpak default behavior.

That's fine here. Afair, this should be only about installing the built ref, there's no requirement that it has to follow Flatpak's default and even if Flatpak refs for runtime are configured at the system level, an user installed ref will be able to share it from system.