Add fallback to detecting main branches for fresh repositories

All other attempts to determine the main branch fail on a fresh
repository right after `git init` because there are no actual commits
that these branches point to.

This does return `master` instead a full name like `refs/heads/master`
which I don't think is an issue, but might be?

Author: Chris McDonnell

pkg/commands/git_commands/main_branches.go

@@ -113,6 +113,17 @@ func (self *MainBranches) determineMainBranches(configuredMainBranches []string)
 				NewGitCmd("rev-parse").Arg("--verify", "--quiet", ref).ToArgv(),
 			).DontLog().Run(); err == nil {
 				existingBranches[i] = ref
+				return
+			}
+
+			// If this is a brand new repository with no commits, then the current branch would be the default
+			if ref, err := self.cmd.New(
+				NewGitCmd("symbolic-ref").Arg("--short", "HEAD").ToArgv(),
+			).DontLog().RunWithOutput(); err == nil {
+				if strings.TrimSpace(ref) == branchName {
+					existingBranches[i] = branchName
+					return
+				}
 			}
 		})
 	}