Make ShowIncludesFilter ignore execroot differences

[meteorcloudy]

Fixes #6847

This change is for making including scanning work on Windows for builds with remote caching or remote execution enabled.

After this change, the ShowIncludesFilter will look for the first execroot\<workspace_name> in the output header file paths, then it considers C:\...\execroot\<workspace_name> as the execroot path. Because execroot path could be different if remote cache is hit, we ignore it and only add the relative path as dependencies.

I'm quite unwilling to make this change, because parsing execroot\\<workspace_name> for execroot is not guaranteed to work always. But considering the only case this could go wrong is when people use an output base that already contains execroot\\<workspace_name>, which I think should never happen.

[laszlocsomor]

But considering the only case this could go wrong is when people use an output base that already contains execroot\<workspace_name>, which I think should never happen.

Could it go wrong if the package path contains that string?

[laszlocsomor]

Could it go wrong if the package path contains that string?

Ah, never mind, the relevant line is the one below, and it only finds the first occurrence: https://github.com/bazelbuild/bazel/pull/6931/files#diff-43621b598be4b035d6872c0b0aa2c44aR74

[meteorcloudy]

Thanks for the fast review!