enhance: replace ${BRANCH} with current branch name if the custom action's scope is Repository (#2274)

Signed-off-by: leo <longshuang@msn.cn>

src/ViewModels/ExecuteCustomAction.cs

@@ -219,7 +219,7 @@ namespace SourceGit.ViewModels
                 Models.Tag t => org.Replace("${TAG}", t.Name),
                 Models.Remote r => org.Replace("${REMOTE}", r.Name),
                 Models.CustomActionTargetFile f => org.Replace("${FILE}", f.File).Replace("${SHA}", f.Revision?.SHA ?? string.Empty),
-                _ => org
+                _ => org.Replace("${BRANCH}", _repo.CurrentBranch?.Name ?? "HEAD")
             };
         }