Fix MR operations for detached heads

[jume-dev]

Description
This fixes a problem with detached heads, where glab tries to find a branch even though an MR ID was given and a branch is not necessary to execute the command.

(also fixes a minor documentation issue I found while trying to setup things to get started)

Related Issue
I can't reopen the issue #735 but as stated there, it was not fixed with the latest release
Resolves #735

How Has This Been Tested?
Manually executing glab mr diff with an ID on a git setup with a detached head

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 #763 (925a61a) into trunk (f62e2fb) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##            trunk     #763   +/-   ##
=======================================
  Coverage   58.47%   58.47%           
=======================================
  Files          93       93           
  Lines        6681     6681           
=======================================
  Hits         3907     3907           
  Misses       2395     2395           
  Partials      379      379           

Impacted Files	Coverage Δ
commands/mr/mrutils/mrutils.go	66.45% <100.00%> (ø)

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[profclems]

Nice catch @jume-dev!

Thanks for fixing this… I’m glad to do a patch release tonight with this fix