Fall back to master if main is missing

Fall back to the `master` configuration regex if `main` is missing from
the configuration. If none of them are found, do a verbatim string
search for branches named `main` with fallback to `master`.

Author: asbjornu

src/GitVersion.Core/Core/RepositoryStore.cs

@@ -128,10 +128,12 @@ public IBranch GetTargetBranch(string? targetBranchName)
 
     public IBranch? FindMainBranch(Config configuration)
     {
-        var mainBranchRegex = configuration.Branches[Config.MainBranchKey]?.Regex;
+        var mainBranchRegex = configuration.Branches[Config.MainBranchKey]?.Regex
+                              ?? configuration.Branches[Config.MasterBranchKey]?.Regex;
+
         if (mainBranchRegex == null)
         {
-            return null;
+            return FindBranch(Config.MainBranchKey) ?? FindBranch(Config.MasterBranchKey);
         }
 
         return this.repository.Branches.FirstOrDefault(b =>