fix(mr): allow boolean flags to explicitly override project defaults

Summary

Fixes three boolean flags (--squash-before-merge, --remove-source-branch, --allow-collaboration) to allow users to explicitly override project defaults by setting them to false.

What was broken

Previously, these flags used plain bool types which couldn't distinguish between:

• User didn't set the flag (should use project default)
• User explicitly set --flag=false (should override project default)

Both cases resulted in the same behavior: the API field was not set, so GitLab used project defaults.

What changed

• Changed three fields from bool to *bool:
  • SquashBeforeMerge
  • RemoveSourceBranch
  • AllowCollaboration
• Updated flag handling in complete() to only set the pointer when the flag is explicitly provided
• Updated run() to check for nil instead of a separate tracking field
• Enhanced flag descriptions to clarify override behavior
• Follows the established pattern from the project update command

Behavior now

For all three flags:

• Flag omitted: API field is nil → GitLab uses project default
• --flag or --flag=true: API field is true → Enable the setting
• --flag=false: API field is false → Disable the setting (overrides project default)

Resolves: #8144 (closed)

Test plan

• Added unit tests covering all three states for all affected flags
• All existing tests pass
• Documentation regenerated