Mac: Verify if VFS for Git detects exchangedata and add functional test

There are not currently any functional tests for exchangedata. Add tests and fix (or file a follow up issue) if VFS for Git does not detect exchangedata.

     int
     exchangedata(const char * path1, const char * path2, unsigned int options);

DESCRIPTION
     The exchangedata() function swaps the contents of the files referenced by path1 and path2 in an atomic fashion.  That is, all concurrent processes will either see the pre-exchanged state or the post-
     exchanged state; they can never see the files in an inconsistent state.  The data in all forks is swapped in this way.  The options parameter lets you control specific aspects of the function's behav-
     iour.


COMPATIBILITY
     Not all volumes support exchangedata().  You can test whether a volume supports exchangedata() by using getattrlist(2) to get the volume capabilities attribute ATTR_VOL_CAPABILITIES, and then testing
     the VOL_CAP_INT_EXCHANGEDATA flag.

No files or existing test names are provided. Locate the VFS for Git functional-test entry point for macOS and the handling of exchangedata, then run the relevant tests. Done means coverage verifies whether exchangedata is detected; if not, either correct the behavior or file a follow-up issue.