Simplify Gradle build logic for making releases.

[blackwinter]

Resolves #684.

Release and release candidate builds require a clean working directory and a matching tag: Create a tag for the release version first, then execute whatever build target you need with the corresponding version specified as project property (-PpublishVersion=A.B.C or -PpublishVersion=A.B.C-rcN). Release builds additionally enforce signing of the generated artifacts.

Ad hoc builds from branches (including master) or arbitrary "version" designators always result in SNAPSHOT builds and don't require a clean working directory or a matching tag: Specify the desired "version" as project property (-PpublishVersion=A-B-C-SNAPSHOT) or simply omit it (will then use the current branch name).

Behavioural changes:

• Release candidates use tags instead of branches, just like regular releases.
• Release candidates require a clean working directory, just like regular releases.
• Publish version must be passed explicitly, it's no longer inferred from repository state and/or tags.
• Ad hoc builds ignore tags altogether (no need to fake a dirty working directory in order to work around the previous build logic).
• Branch names don't carry any meaning (previously releases/* for release builds and *-rc* for release candidates).
• Aborts the build instead of warning/skipping if any preconditions aren't met (i.e. working directory state, matching tag).

[dr0i]

Besides he empty line I think this is good 👍

[TobiasNx]

@blackwinter can you help me how to review this? what should I try to run?

[blackwinter]

First of all, you should review the updated steps in MAINTAINING.md. Do they make sense to you? Are they sufficiently simple, fool-proof, without providing too many opportunities for mistakes?

Then you could run various scenarios (without actually pushing/publishing anything!). Different branches/detached head, different publishVersions, with and without providing the required tags (which should be cleaned up afterwards!). The most basic task would be clean which then just prints the version; you could also generate the distribution with assemble.

If you need more guidance we should meet for a "review session".