Callback to perform the merge.

Specified as driver.apply, this is the callback that actually does the merge. If it can successfully perform a merge, it should populate path_out with a pointer to the filename to accept, mode_out with the resultant mode, and merged_out with the buffer of the merged file and then return 0. If the driver returns GIT_PASSTHROUGH, then the default merge driver should instead be run. It can also return GIT_EMERGECONFLICT if the driver is not able to produce a merge result, and the file will remain conflicted. Any other errors will fail and return to the caller.

The filter_name contains the name of the filter that was invoked, as specified by the file's attributes.

The src contains the data about the file to be merged.