feat(mr update): add --squash-before-merge parameter

[pataar]

Description

This adds the --squash-before-merge parameter to mr update. Which allows the merge request the be squashed by default.

A point of discussion: I thought it would be practical that this would toggle the option to squash. Not sure what your preference has, @profclems. If you'd like it differently, please let me know!

See: docs.gitlab.com/ee/api/merge_requests.html#update-mr

How Has This Been Tested?

I've used the binary built from this fork against a private Gitlab instance. Worked as expected.

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 #856 (4170736) into trunk (2e4c14b) will increase coverage by 0.00%.
The diff coverage is n/a.

@@           Coverage Diff           @@
##            trunk     #856   +/-   ##
=======================================
  Coverage   59.31%   59.31%           
=======================================
  Files          93       93           
  Lines        7383     7384    +1     
=======================================
+ Hits         4379     4380    +1     
  Misses       2619     2619           
  Partials      385      385           

Impacted Files	Coverage Δ
commands/ci/ci.go	100.00% <0.00%> (ø)

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[pataar]

Fixed the pipelines!