Checkout button needs a downwards-facing triangle to indicate choice from a pop-up menu in GtGitCommit

[StephanEggermont]

Checkout now looks like an action button, but shows three possible actions:

• Checkout packages and Lepiter databases already loaded in the image
• Checkout all packages in the repository
• Do not checkout any packages, I will load them manually (for experts only)

Default differentiator for that is a text color downwards-facing filled triangle.

Found while trying to resolve a detached head in 1.0.633 Windows11