Handle source build patches

[premun]

Context

In the dotnet/installer repo, we have a list of file patches that we apply onto the files when creating the tarball.

In the VMR, we have several ways how to deal with this, e.g. have patches already applied in the VMR or applying them during the source-build. There are benefits and drawbacks to each approach as we want to keep track of the patches.

Goal

We need to consider how to bake this step into the VMR so that we deal with these in some maintainable fashion.

[premun]

We have decided that:

• Files are checked into the VMR with patches already applied onto them.
• Patches are also part of the VMR for bookkeeping. Furthermore, if we put them directly in the VMR so that conflicts in patches can be dealt with in PRs in the VMR.

Consider we're updating repo A from sha1 to sha2, the algorithm I propose is as follows:

1. Checkout VMR
2. Clone individual repo A to temp
3. For each file that has a patch in the VMR, copy its sha1 version from the clone to the VMR (to working tree only)
4. Prepare a diff for sha1..sha2 from the clone and apply it
5. Apply the VMR patches
6. Commit all above to the VMR

Here is a proposal on how we could deal with various situations around SB patches in VMR-lite:
https://microsofteur-my.sharepoint.com/:u:/g/personal/prvysoky_microsoft_com/EedRj0vobDRKrh054Wco85oBW2iLBF3vZRPz6Mmq-tKGnw?e=Yii1r6

The breakdown of how the sync will work in different situation is as follows:

[premun]

Seems like I can do this to get a list of files that are changed by a patch using git apply --numstat:

$ git apply --numstat --directory src/roslyn-analyzers/ src/installer/src/SourceBuild/tarball/patches/roslyn-analyzers/0001-Eliminate-pre-built-assets-during-source-build-for-r.patch
0       14      src/roslyn-analyzers/eng/Versions.props
3       2       src/roslyn-analyzers/src/Directory.Build.targets
1       1       src/roslyn-analyzers/src/Microsoft.CodeAnalysis.Analyzers/Core/MetaAnalyzers/Fixers/PreferIsKindFix.cs
0       1       src/roslyn-analyzers/src/Microsoft.CodeAnalysis.ResxSourceGenerator/Microsoft.CodeAnalysis.ResxSourceGenerator.CSharp/Microsoft.CodeAnalysis.ResxSourceGenerator.CSharp.csproj
0       1       src/roslyn-analyzers/src/Microsoft.CodeAnalysis.ResxSourceGenerator/Microsoft.CodeAnalysis.ResxSourceGenerator.VisualBasic/Microsoft.CodeAnalysis.ResxSourceGenerator.VisualBasic.csproj
0       1       src/roslyn-analyzers/src/Microsoft.CodeAnalysis.ResxSourceGenerator/Microsoft.CodeAnalysis.ResxSourceGenerator/Microsoft.CodeAnalysis.ResxSourceGenerator.csproj
1       1       src/roslyn-analyzers/src/NetAnalyzers/CSharp/Microsoft.NetCore.Analyzers/InteropServices/CSharpDisableRuntimeMarshalling.FixAllProvider.cs
1       1       src/roslyn-analyzers/src/NetAnalyzers/Core/Microsoft.CodeQuality.Analyzers/ApiDesignGuidelines/DoNotDirectlyAwaitATask.Fixer.cs
1       1       src/roslyn-analyzers/src/Roslyn.Diagnostics.Analyzers/CSharp/Roslyn.Diagnostics.CSharp.Analyzers.csproj
0       1       src/roslyn-analyzers/src/Roslyn.Diagnostics.Analyzers/Core/Roslyn.Diagnostics.Analyzers.csproj
1       1       src/roslyn-analyzers/src/Roslyn.Diagnostics.Analyzers/VisualBasic/Roslyn.Diagnostics.VisualBasic.Analyzers.vbproj
0       1       src/roslyn-analyzers/src/Tools/GenerateDocumentationAndConfigFiles/GenerateDocumentationAndConfigFiles.csproj
1       3       src/roslyn-analyzers/src/Tools/GenerateDocumentationAndConfigFilesForBrokenRuntime/GenerateDocumentationAndConfigFilesForBrokenRuntime.csproj
1       0       src/roslyn-analyzers/src/Tools/ReleaseNotesUtil/ReleaseNotesUtil.csproj

Somehow --summary didn't work for me on the Ubuntu VM (I have the newest git) which is a shame because that one contains also change mode (add, change, remove..).