Fix glab repo view handling of branches

[brodock]

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

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)

[codecov]

Codecov Report

Merging #787 (14ba6cf) into trunk (8bd042a) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##            trunk     #787   +/-   ##
=======================================
  Coverage   58.34%   58.34%           
=======================================
  Files          93       93           
  Lines        6696     6696           
=======================================
  Hits         3907     3907           
  Misses       2409     2409           
  Partials      380      380           

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