Fix revision metadata in Git worktrees

[ar-agt]

What Problem This Solves

Any Gradle build invoked from a linked Git worktree fails during task input fingerprinting because the revision task assumes .git is a directory and dereferences a missing .git/HEAD.

Why This Change Was Made

• resolve .git when it is either a directory or a worktree gitdir: pointer
• resolve symbolic branch refs through the worktree's commondir
• make the optional ref input null-safe
• preserve detached-HEAD metadata and the existing unknown/unknown fallback for exported source

This uses Git's metadata files directly, without requiring the git executable or adding a dependency.

User Impact

jPOS can be built normally from linked Git worktrees, and generated revision.properties contains the correct branch/revision metadata. Builds from source exports without .git continue to work with unknown revision metadata.

Evidence

• ./gradlew :jpos:compileJava --no-daemon from a linked worktree
• ./gradlew jpos:check --no-daemon
• verified createRevisionPropertyFile in an ordinary checkout, a branch worktree, a detached worktree, and a source export without .git

Fixes #757