feat(commands/mr/checkout): add --set-upstream-to flag

[maxice8]

Description

This adds the --set-upstream-to | -u flag which denotes a [REMOTE/]BRANCH to track.

This is mutually exclusive with --track because that also sets a tracking.

Related Issue

Resolves #544

How Has This Been Tested?

glab mr checkout rspamd --set-upstream-to upstream/master

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 #545 (85a745b) into trunk (54324ab) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##            trunk     #545   +/-   ##
=======================================
  Coverage   60.88%   60.88%           
=======================================
  Files          86       86           
  Lines        5913     5913           
=======================================
  Hits         3600     3600           
  Misses       1979     1979           
  Partials      334      334           

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