Incremental build is broken for external packages with patches

[ViktorHofer]

source-build-reference-packages/src/externalPackages/projects/Directory.Build.targets

Lines 15 to 47 in a2143ed

	<Target Name="PrepareInnerClone">
	<ItemGroup>
	<RepoFileToCopy Include="$(OriginalProjectDirectory)**\*"
	Exclude="$(OriginalProjectDirectory)artifacts\**\*" />
	</ItemGroup>
	<Copy SourceFiles="@(RepoFileToCopy)" DestinationFolder="$(ProjectDirectory)%(RecursiveDir)" SkipUnchangedFiles="true" />
	</Target>
	<Target Name="ApplyPatches"
	Condition="'@(PatchToApply)' != '' and '$(SkipPatches)' != 'true'"
	DependsOnTargets="PrepareInnerClone"
	Inputs="@(PatchToApply)"
	Outputs="$(BaseIntermediateOutputPath)ApplyPatches.complete" >
	<PropertyGroup>
	<PatchCommand>git --work-tree=$(ProjectDirectory) apply --ignore-whitespace --whitespace=nowarn</PatchCommand>
	</PropertyGroup>
	<!-- in the installer tarball, we want to remove the objects directory from the .gitdir to save space.
	This causes a problem specifically in the combination of submodules which have redirected .gitdirs
	when we are applying patches in the current directory (which is required due to the way Git
	interprets the paths in a patch). GIT_DIR=/dev/null short-circuits the .gitdir discovery process
	and lets Git treat the directory like any non-Git-controlled directory instead. -->
	<Exec Command="$(PatchCommand) %(PatchToApply.Identity)"
	WorkingDirectory="$(ProjectDirectory)"
	Condition="'@(PatchToApply)' != ''"
	EnvironmentVariables="GIT_DIR=/dev/null" />
	<MakeDir Directories="$(BaseIntermediateOutputPath)" />
	<Touch Files="$(BaseIntermediateOutputPath)ApplyPatches.complete" AlwaysCreate="true">
	<Output TaskParameter="TouchedFiles" ItemName="FileWrites" />
	</Touch>
	</Target>

1. The PrepareInnerClone target always runs but skips unchanged files.
2. The ApplyPatches target which runs after PrepareInnerClone only runs once successfully as it then creates an output marker file.

If something fails in the SBRP repo and I run the build again, the PrepareInnerClone target runs again and overwrites the patched files but the ApplyPatches target doesn't run anymore.

[MichaelSimons]

This appears to be the same issue described in #1310.

[ViktorHofer]

I would probably just remove the Inputs & Outputs from the target that applies the patches. That means that incremental builds are not really incremental for SBRP components but better than the current situation. We need to always copy files as otherwise the "clone" directory from which we build the submodule doesn't receive code edits.