Use explicit variable for OptProf source branch (#4295)
Use explicit variable for OptProf source branch (#4295)

When the source branch was hardcoded to `$(Build.SourceBranchName)`,
there was no good way to tell the build "build this branch, but take the
latest OptProf information from branch X". That's an important scenario
when standing up a new branch (like a release branch), or trying to
insert updated bits from a feature branch.

This requires a change to the build definition, which is the only place
(currently) that knows what variables to offer at job-scheduling time.
mono/msbuild,AndyGerlicher/msbuild,rainersigwald/msbuild,cdmihai/msbuild,chipitsine/msbuild,jeffkl/msbuild,AndyGerlicher/msbuild,chipitsine/msbuild,jeffkl/msbuild,cdmihai/msbuild,jeffkl/msbuild,sean-gilliam/msbuild,sean-gilliam/msbuild,cdmihai/msbuild,sean-gilliam/msbuild,Microsoft/msbuild,jeffkl/msbuild,Microsoft/msbuild,rainersigwald/msbuild,sean-gilliam/msbuild,Microsoft/msbuild,mono/msbuild,Microsoft/msbuild,chipitsine/msbuild,AndyGerlicher/msbuild,AndyGerlicher/msbuild,cdmihai/msbuild,rainersigwald/msbuild,rainersigwald/msbuild,chipitsine/msbuild,mono/msbuild,mono/msbuild