Troubleshoot backport failures

Lately we see a lot of backport workflows fail. The pattern always
seems to be that the first backport (to branch/v17) succeeds, but
subsequent backports (to v15 and v16) almost always fail.

The failure looks to be a GitHub-specific failure, not a generic git
failure:

    remote: fatal error in commit_refs
      To https://github.com/gravitational/teleport
      ! [remote rejected]       bot/backport-51989-branch/v15 > bot/backport-51989-branch/v15 (failure)
    error: failed to push some refs to https://github.com/gravitational/teleport

To aid in troubleshooting we:
1. Temporarily reverse the order of branches. This will tell us if it's
   always the first branch that succeeds, or if it's always branch/v17
   that succeeds.
2. Add the --verbose flag to the git push command in case we get any
   more debug info.

bot/internal/bot/backport.go

@@ -84,6 +84,10 @@ func (b *Bot) Backport(ctx context.Context) error {
 		g = b.c.Git
 	}
 
+	// Temporary experiment to change the order of backport attempts.
+	// TODO(zmb3): remove this
+	slices.Reverse(branches)
+
 	// Loop over all requested backport branches and create backport branch and
 	// GitHub Pull Request.
 	for _, base := range branches {
@@ -249,7 +253,7 @@ func (b *Bot) createBackportBranch(ctx context.Context, organization string, rep
 	}
 
 	// Push the backport branch to Github.
-	if err := git("push", "origin", newHead); err != nil {
+	if err := git("push", "--verbose", "origin", newHead); err != nil {
 		return trace.Wrap(err)
 	}