Use remote upstreams of main branches to determine merged status of commits

This solves three problems:

1. When the local main branch is behind its upstream, the merged status of
   commits of a feature branch sitting on origin/main was not correct. This can
   easily happen when you rebase a branch onto origin/main instead of main, and
   don't bother keeping local main up to date.
2. It works when you don't have the main branch locally at all. This could
   happen when you check out a colleague's feature branch that goes off of
   "develop", but you don't have "develop" locally yourself because you normally
   only work on "main".
3. It also works when you work on a main branch itself, e.g. by committing to it
   directly, or by merging a branch locally. These local commits on a main
   branch would previously be shown in green instead of red; this broke with
   910a61dc46.

pkg/commands/git_commands/commit_loader.go

@@ -490,13 +490,33 @@ func (self *CommitLoader) getMergeBase(refName string) string {
 func (self *CommitLoader) getExistingMainBranches() []string {
 	return lo.FilterMap(self.UserConfig.Git.MainBranches,
 		func(branchName string, _ int) (string, bool) {
-			ref := "refs/heads/" + branchName
+			// Try to determine upstream of local main branch
+			if ref, err := self.cmd.New(
+				NewGitCmd("rev-parse").Arg("--symbolic-full-name", branchName+"@{u}").ToArgv(),
+			).DontLog().RunWithOutput(); err == nil {
+				return strings.TrimSpace(ref), true
+			}
+
+			// If this failed, a local branch for this main branch doesn't exist or it
+			// has no upstream configured. Try looking for one in the "origin" remote.
+			ref := "refs/remotes/origin/" + branchName
 			if err := self.cmd.New(
 				NewGitCmd("rev-parse").Arg("--verify", "--quiet", ref).ToArgv(),
-			).DontLog().Run(); err != nil {
-				return "", false
+			).DontLog().Run(); err == nil {
+				return ref, true
 			}
-			return ref, true
+
+			// If this failed as well, try if we have the main branch as a local
+			// branch. This covers the case where somebody is using git locally
+			// for something, but never pushing anywhere.
+			ref = "refs/heads/" + branchName
+			if err := self.cmd.New(
+				NewGitCmd("rev-parse").Arg("--verify", "--quiet", ref).ToArgv(),
+			).DontLog().Run(); err == nil {
+				return ref, true
+			}
+
+			return "", false
 		})
 }