When a branch name with special characters like % is passed to the RepositoryService.GetBranch() method as is, an error is currently returned:

invalid URL escape "%"

Passing an escaped branch name to RepositoriesService.GetBranch() works fine.

Here's a minimal reproducer

import (
	"context"
	"net/http"
	"testing"

	"github.com/google/go-github/v53/github"
	"github.com/stretchr/testify/require"
)

func TestGetBranch(t *testing.T) {
	t.Parallel()
	ctx := context.Background()

	githubClient := github.NewClient(&http.Client{
		Transport: http.DefaultTransport,
	})
	owner := "samstickkz"
	project := "curr"
	branchName := "Feature/authentication_90%"
	branch, response, err := githubClient.Repositories.GetBranch(ctx, owner, project, branchName, false)

	require.Nil(t, err)
	t.Logf("Response: %+v", response)
	t.Logf("Branch: %+v", branch)
}

This sounds like a bug to me, as I believe users of that method shouldn't be expected to escape the branch name before calling it, as this is a private implementation quirk that they shouldn't be aware of. This PR patches all branch-related methods to automatically escape the branch name in the endpoint URL.

There's actually a precedent for ref parameter, and I suspect that other variables are prone to the same issue. I am conservatively patching only the branch variable, as this is the only one I have tested thoroughly. Note that, contrary to ref use case, my tests suggest that it's okay to escape / characters for branch names.

⚠️ Breaking change: for users that currently bypass this issue by feeding escaped strings to RepositoryService.GetBranch() method (and other branch-related methods), this change will be breaking.