Fix `glab repo view` handling of branches

Description This does fix a few things in glab repo view command:

1. Uses provided -b branchname to also retrieve the README file.
2. Only override an empty opts.Branch when a repository is not provided as argument
3. The generated URL for opening in a browser only appends /-/tree/ when provided branch is not an empty string

Related Issue Resolves #786 (closed)

How Has This Been Tested?

Screenshots (if appropriate):

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation
• Chore (Related to CI or Packaging to platforms)