enhance GetCurrentBranch for drone

src/GitVersionCore/BuildServers/Drone.cs

@@ -24,6 +24,31 @@ public override string[] GenerateSetParameterMessage(string name, string value)
 
         public override string GetCurrentBranch(bool usingDynamicRepos)
         {
+            // In Drone DRONE_BRANCH variable is equal to destination branch in case of pull request
+            // https://discourse.drone.io/t/getting-the-branch-a-pull-request-is-created-from/670
+            // Unfortunately, DRONE_REFSPEC isn't populated, however CI_COMMIT_REFSPEC can be used instead of.
+            var pullRequestNumber = Environment.GetEnvironmentVariable("DRONE_PULL_REQUEST");
+            if (!string.IsNullOrWhiteSpace(pullRequestNumber))
+            {
+                // DRONE_SOURCE_BRANCH is available in Drone 1.x.x version
+                var sourceBranch = Environment.GetEnvironmentVariable("DRONE_SOURCE_BRANCH");
+                if (!string.IsNullOrWhiteSpace(sourceBranch))
+                    return sourceBranch;
+
+                // In drone lower than 1.x.x source branch can be parsed from CI_COMMIT_REFSPEC
+                // CI_COMMIT_REFSPEC - {sourceBranch}:{destinationBranch}
+                // https://github.com/drone/drone/issues/2222
+                var ciCommitRefSpec = Environment.GetEnvironmentVariable("CI_COMMIT_REFSPEC");
+                if (!string.IsNullOrWhiteSpace(ciCommitRefSpec))
+                {
+                    var colonIndex = ciCommitRefSpec.IndexOf(':');
+                    if (colonIndex > 0)
+                    {
+                        return ciCommitRefSpec.Substring(0, colonIndex);
+                    }
+                }
+            }
+
             return Environment.GetEnvironmentVariable("DRONE_BRANCH");
         }
     }