Two additional heuristics for choosing mainline

When there is more than one potential mainline branch, before falling
back to the first branch in the list, choose 1) the current branch, if
it is a possible mainline; or 2) the first potential mainline from which
the current commit is reachable in a FirstParentOnly walk (i.e. is a
direct commit on that branch).

Additionally, don't discard potential mainlines with equal tip because
it might reduce the effectiveness of the new heuristics.

Fixes the test MainlineDevelopmentMode.VerifySupportForwardMerge vs the
previous commit.

Author: wjrogers

src/GitVersionCore/VersionCalculation/MainlineVersionCalculator.cs

@@ -34,7 +34,7 @@ public SemanticVersion FindMainlineModeVersion(BaseVersion baseVersion, GitVersi
                 // 
 
                 var mergeBase = baseVersion.BaseVersionSource;
-                var mainline = GetMainline(context);
+                var mainline = GetMainline(context, baseVersion.BaseVersionSource);
                 var mainlineTip = mainline.Tip;
 
                 // when the current branch is not mainline, find the effective mainline tip for versioning the branch
@@ -101,25 +101,14 @@ SemanticVersion AggregateMergeCommitIncrement(GitVersionContext context, Commit
             return mainlineVersion;
         }
 
-        static Branch GetMainline(GitVersionContext context)
+        static Branch GetMainline(GitVersionContext context, Commit baseVersionSource)
         {
             var mainlineBranchConfigs = context.FullConfiguration.Branches.Where(b => b.Value.IsMainline == true).ToList();
-            var seenMainlineTips = new List<string>();
             var mainlineBranches = context.Repository.Branches
                 .Where(b =>
                 {
                     return mainlineBranchConfigs.Any(c => Regex.IsMatch(b.FriendlyName, c.Value.Regex));
                 })
-                .Where(b =>
-                {
-                    if (seenMainlineTips.Contains(b.Tip.Sha))
-                    {
-                        Logger.WriteInfo("Multiple possible mainlines pointing at the same commit, dropping " + b.FriendlyName);
-                        return false;
-                    }
-                    seenMainlineTips.Add(b.Tip.Sha);
-                    return true;
-                })
                 .Select(b => new
                 {
                     MergeBase = context.Repository.ObjectDatabase.FindMergeBase(b.Tip, context.CurrentCommit),
@@ -143,6 +132,38 @@ static Branch GetMainline(GitVersionContext context)
                 return mainlineBranch;
             }
 
+            // prefer current branch, if it is a mainline branch
+            if (possibleMainlineBranches.Any(context.CurrentBranch.IsSameBranch))
+            {
+                Logger.WriteInfo(string.Format("Choosing {0} as mainline because it is the current branch", context.CurrentBranch.FriendlyName));
+                return context.CurrentBranch;
+            }
+
+            // prefer a branch on which the merge base was a direct commit, if there is such a branch
+            var firstMatchingCommitBranch = possibleMainlineBranches
+                .FirstOrDefault(b =>
+                {
+                    var filter = new CommitFilter
+                    {
+                        IncludeReachableFrom = b,
+                        ExcludeReachableFrom = baseVersionSource,
+                        FirstParentOnly = true,
+                    };
+                    var query = context.Repository.Commits.QueryBy(filter);
+
+                    return query.Contains(firstMatchingCommit);
+                });
+            if (firstMatchingCommitBranch != null)
+            {
+                var message = string.Format(
+                    "Choosing {0} as mainline because {1}'s merge base was a direct commit to {0}",
+                    firstMatchingCommitBranch.FriendlyName,
+                    context.CurrentBranch.FriendlyName);
+                Logger.WriteInfo(message);
+
+                return firstMatchingCommitBranch;
+            }
+
             var chosenMainline = possibleMainlineBranches[0];
             Logger.WriteInfo(string.Format(
                 "Multiple mainlines ({0}) have the same merge base for the current branch, choosing {1} because we found that branch first...",