fix: try to update ref head only if remote has diverged

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Description

If the remote has diverged from the local branches, glab mr checkout will fail.

the cause is during checkout, the cli invokes git fetch git@gitlab.com/xxx/yyy.git refs/heads/branchA:branchA trying to update the local ref to the remote.

but this happens if the remote has diverged

! [rejected]        branchA -> branchA  (non-fast-forward)

the proposed change retries git fetch git@gitlab.com/xxx/yyy.git refs/heads/branchA that only update the ref head without updating the branch head and leave the user to resolve the divergence between the remote and local branches

Related Issues

Resolves #7863 (closed)

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)
• Test gap