Add ThisAssembly.Git

[kzu]

This is something that GitInfo provides, but it does so using MSBuild and it seems to cause an issue in some source-generator scenarios.

So perhaps it would be best if the codegen part was provided by a source generator in this project instead, with GitInfo detecting its presence (via a ThisAssemblyGitInfo=true property perhaps?) and avoid generating the ThisAssembly.Git part of the code.

It might even make sense for GitInfo to take a dependency on the new ThisAssembly.Git entirely and just augment it for the semver determination.

This implementation should likely rely on SourceLink implementation for some information retrieval, and take a dependency on it to avoid duplicating work?

[viceroypenguin]

The biggest problem with doing this as a source generator is that SG should not do I/O (in fact, recommendation is that no SG should take more than 10ms to complete). Getting git info would require doing I/O to open the files .git uses to read the data it needs. Recommend that the current path (build task in GitInfo) is the proper way to do this.

[kzu]

SourceLink already populates most (all?) of the information GitInfo calculates. So perhaps it's mostly just exposing those to .NET code...

[viceroypenguin]

To make sure I understand: the SG would not store the values, simply the code that access the data present in the SourceLink area?

[kzu]

Yep, that's the idea... to avoid redoing anything that is already solved by sourcelink itself.. Perhaps we could add reading of the RepositoryBranch (which is a valid metadata item for nuget pack) (perhaps just by reading the HEAD from MSBuild and making it visible to the SG...