Load auto-mode on showing revision from git.
Load auto-mode on showing revision from git.

This requires some dubious hacking of file-name internals for a
buffer, but seems to work.

Also significantly clean up / improve the discovery of the current
source file name.

Line jumping for to new version of the source still needs significant
work.

tomheon/diffscuss,hut8labs/diffscuss,hut8labs/diffscuss,tomheon/diffscuss