Pull event handler should skip actioning unmapped files

[isc-tleavitt]

e.g., things that are in the repo for safekeeping but not actually source controlled - the risk is that stray things in the repo could be imported and overwrite changes that are not reflected in the repo automatically

[isc-tleavitt]

This is a common use case in HCC for e.g. System Default Settings, which are present in all branches but in a branch-specific folder.

[isc-tleavitt]

The error here is actually that we'd hit a error trying to add a new unmapped file to the source control file tracking global.

[isc-tleavitt]

(looking at better logging and trying to run a bunch of edge cases for #467 this popped out with a bit more clarity.)