Initial fix for issues around merging to a feature branch and back to master

Author: Mark Rydstrom

src/GitVersionCore/VersionCalculation/NextVersionCalculator.cs

@@ -101,6 +101,15 @@ SemanticVersion FindMainlineModeVersion(BaseVersion baseVersion, GitVersionConte
                     ExcludeReachableFrom = baseVersion.BaseVersionSource,
                     SortBy = CommitSortStrategies.Reverse
                 }).Where(c => c.Sha != baseVersion.BaseVersionSource.Sha).ToList();
+
+                var firstParentLog = context.Repository.Commits.QueryBy(new CommitFilter
+                {
+                    IncludeReachableFrom = context.CurrentBranch,
+                    ExcludeReachableFrom = baseVersion.BaseVersionSource,
+                    SortBy = CommitSortStrategies.Reverse,
+                    FirstParentOnly = true
+                }).Where(c => c.Sha != baseVersion.BaseVersionSource.Sha).ToList();
+
                 var directCommits = new List<Commit>();
 
                 // Scans commit log in reverse, aggregating merge commits
@@ -109,7 +118,14 @@ SemanticVersion FindMainlineModeVersion(BaseVersion baseVersion, GitVersionConte
                     directCommits.Add(commit);
                     if (commit.Parents.Count() > 1)
                     {
-                        mainlineVersion = AggregateMergeCommitIncrement(context, commit, directCommits, mainlineVersion);
+                        if (firstParentLog.Contains(commit))
+                        {
+                            mainlineVersion = AggregateMergeCommitIncrement(context, commit, directCommits, mainlineVersion);
+                        }
+                        else
+                        {
+                            directCommits.Remove(commit);
+                        }
                     }
                 }