Provide RepositoryBranch metadata for package

As documented in nuget (https://learn.microsoft.com/en-us/nuget/reference/msbuild-targets) you can provide
not only the repo URL, type and commit, but also the branch. But this information is not populated by default
by the SourceLink package, neither the ReproducibleBuilds (unless the PR at dotnet/reproducible-builds#27 is merged).

So just bring the code in that PR here until that's merged and shipped.

Fixes Sergio0694#23

Directory.Build.props

@@ -85,7 +85,38 @@
     <GenerateDocumentationFile>true</GenerateDocumentationFile>
   </PropertyGroup>
 
+  <PropertyGroup Condition="'$(RepositoryBranch)' == ''">
+    <!-- GitHub Actions: https://docs.github.com/en/actions/reference/environment-variables#default-environment-variables -->
+    <RepositoryBranch Condition="'$(RepositoryBranch)' == '' and '$(GITHUB_REF)' != '' and $(GITHUB_REF.Contains('refs/pull/'))">pr$(GITHUB_REF.Replace('refs/pull/', '').Replace('/merge', ''))</RepositoryBranch>
+    <RepositoryBranch Condition="'$(RepositoryBranch)' == '' and '$(GITHUB_REF)' != ''">$(GITHUB_REF.Replace('refs/heads/', '').Replace('refs/tags/', ''))</RepositoryBranch>
+    <!-- Azure DevOps: https://docs.microsoft.com/en-us/azure/devops/pipelines/build/variables -->
+    <RepositoryBranch Condition="'$(RepositoryBranch)' == '' and '$(BUILD_SOURCEBRANCH)' != ''">$(BUILD_SOURCEBRANCH.Replace('refs/heads/', '').Replace('refs/tags/', ''))</RepositoryBranch>
+    <!-- AppVeyor: https://www.appveyor.com/docs/environment-variables/ -->
+    <RepositoryBranch Condition="'$(RepositoryBranch)' == '' and '$(APPVEYOR_PULL_REQUEST_NUMBER)' != ''">pr$(APPVEYOR_PULL_REQUEST_NUMBER)</RepositoryBranch>
+    <RepositoryBranch Condition="'$(RepositoryBranch)' == '' and '$(APPVEYOR_REPO_TAG_NAME)' != ''">$(APPVEYOR_REPO_TAG_NAME)</RepositoryBranch>
+    <RepositoryBranch Condition="'$(RepositoryBranch)' == '' and '$(APPVEYOR_REPO_BRANCH)' != ''">$(APPVEYOR_REPO_BRANCH)</RepositoryBranch>
+    <!-- TeamCity: https://www.jetbrains.com/help/teamcity/predefined-build-parameters.html#Branch-Related+Parameters -->
+    <RepositoryBranch Condition="'$(RepositoryBranch)' == '' and '$(TEAMCITY_BUILD_BRANCH)' != ''">$(TEAMCITY_BUILD_BRANCH)</RepositoryBranch>
+    <!--TravisCI: https://docs.travis-ci.com/user/environment-variables/ -->
+    <RepositoryBranch Condition="'$(RepositoryBranch)' == '' and '$(TRAVIS_PULL_REQUEST)' != '' and '$(TRAVIS_PULL_REQUEST)' != 'false'">pr$(TRAVIS_PULL_REQUEST)</RepositoryBranch>
+    <RepositoryBranch Condition="'$(RepositoryBranch)' == '' and '$(TRAVIS_BRANCH)' != ''">$(TRAVIS_BRANCH)</RepositoryBranch>
+    <!-- CircleCI: https://circleci.com/docs/2.0/env-vars/ -->
+    <RepositoryBranch Condition="'$(RepositoryBranch)' == '' and '$(CIRCLE_PR_NUMBER)' != ''">pr$(CIRCLE_PR_NUMBER)</RepositoryBranch>
+    <RepositoryBranch Condition="'$(RepositoryBranch)' == '' and '$(CIRCLE_TAG)' != ''">$(CIRCLE_TAG)</RepositoryBranch>
+    <RepositoryBranch Condition="'$(RepositoryBranch)' == '' and '$(CIRCLE_BRANCH)' != ''">$(CIRCLE_BRANCH)</RepositoryBranch>
+    <!-- GitLab: https://docs.gitlab.com/ee/ci/variables/predefined_variables.html -->
+    <RepositoryBranch Condition="'$(RepositoryBranch)' == '' and '$(CI_COMMIT_TAG)' != ''">$(CI_COMMIT_TAG)</RepositoryBranch>
+    <RepositoryBranch Condition="'$(RepositoryBranch)' == '' and '$(CI_MERGE_REQUEST_IID)' != ''">pr$(CI_MERGE_REQUEST_IID)</RepositoryBranch>
+    <RepositoryBranch Condition="'$(RepositoryBranch)' == '' and '$(CI_EXTERNAL_PULL_REQUEST_IID)' != ''">pr$(CI_EXTERNAL_PULL_REQUEST_IID)</RepositoryBranch>
+    <RepositoryBranch Condition="'$(RepositoryBranch)' == '' and '$(CI_COMMIT_BRANCH)' != ''">$(CI_COMMIT_BRANCH)</RepositoryBranch>
+    <!-- Buddy: https://buddy.works/docs/pipelines/environment-variables#default-environment-variables -->
+    <RepositoryBranch Condition="'$(RepositoryBranch)' == '' and '$(BUDDY_EXECUTION_PULL_REQUEST_NO)' != ''">pr$(BUDDY_EXECUTION_PULL_REQUEST_NO)</RepositoryBranch>
+    <RepositoryBranch Condition="'$(RepositoryBranch)' == '' and '$(BUDDY_EXECUTION_TAG)' != ''">$(BUDDY_EXECUTION_TAG)</RepositoryBranch>
+    <RepositoryBranch Condition="'$(RepositoryBranch)' == '' and '$(BUDDY_EXECUTION_BRANCH)' != ''">$(BUDDY_EXECUTION_BRANCH)</RepositoryBranch>
+  </PropertyGroup>    
+
   <ItemGroup>
+  <!-- Delete the previous property group when https://github.com/dotnet/reproducible-builds/pull/27 ships -->
     <PackageReference Include="DotNet.ReproducibleBuilds" Version="1.1.1" PrivateAssets="all" />
   </ItemGroup>