allow more than one merge request to be defined for certain mr commands

[maxice8]

Description
Certain commands from mr now support passing multiple merge request via multiple args like:

foo@bar:~$ glab mr close 1 branch-that-maps-to-mr-2 3
[closes MRs !1 !2 !3]

How Has This Been Tested?

Tested by approving multiple merge requests and then revoking them on gitlab.alpinelinux.org

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)

[profclems]

Sorry I took long reviewing this PR. It touches crucial parts and I had to make sure everything is in place...
Everything looks good but the test should be fixed

[codecov]

Codecov Report

Merging #622 (a5314ba) into trunk (0be545a) will increase coverage by 0.01%.
The diff coverage is 62.06%.

@@            Coverage Diff             @@
##            trunk     #622      +/-   ##
==========================================
+ Coverage   60.49%   60.51%   +0.01%     
==========================================
  Files          90       90              
  Lines        6354     6357       +3     
==========================================
+ Hits         3844     3847       +3     
+ Misses       2149     2148       -1     
- Partials      361      362       +1     

Impacted Files	Coverage Δ
commands/mr/mrutils/mrutils.go	79.50% <0.00%> (+4.89%)	⬆️
commands/mr/subscribe/mr_subscribe.go	81.25% <66.66%> (ø)
commands/mr/unsubscribe/mr_unsubscribe.go	81.25% <66.66%> (ø)
commands/mr/delete/mr_delete.go	84.61% <100.00%> (+0.61%)	⬆️
commands/issue/create/issue_create.go	32.12% <0.00%> (-0.58%)	⬇️

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[profclems]

Thanks for taking this on @maxice8