Add option to get Git commit information through C API

[maxrjones]

Description of proposed changes

This PR adds API_GIT_COMMIT to the API keywords for GMT_Get_Default, so that wrappers can more easily access the commit information for the GMT library (xref GenericMappingTools/pygmt#1838).

Fixes #

Reminders

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• Add tests for new features or tests that would have caught the bug that you're fixing.
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[seisman]

I'm wondering if API_BIN_VERSION is a better name here, because for public releases, it's a version string like 6.4.0 without git commit information.

[seisman]

Also need to update the API documentation https://docs.generic-mapping-tools.org/dev/devdocs/api.html#get-or-set-an-api-or-gmt-default-parameter.