Cache gitVersion() result

[robert3005]

if you do

allprojects {
  version gitVersion()
}

you will compute version for every subproject even though they're in the same repo. This increases the runtime by around ~0.5s per subproject. We should cache the result since it won't change across projects.

[ddimtirov]

We should cache the result since it won't change across projects.

It is a bit more complex than this. Consider the following use 2 cases:

• some composite projects may reside in multiple repos, and will have different versions
  • in such case we can cache the version per repo-root.
• the build may create tags internally
  • which means that we need a way to invalidate the cache
  • ideally there should be a cheap check (cheaper than describing the tag) to detect that the repo has been modified and invalidate automatically