[mythtv] FFmpeg plans

[Paul Gardiner]

There seems to be a little confusion concerning how git subtree works. 
It doesn't do anything weird to the subdirectory it's applied to: the 
only difference anyone would notice in the mythtv repo, if using git 
subtree, is that the updates to the ffmpeg subdir would appear as merges 
and some of the commits in the branches merged would mysteriously refer 
to files within the ffmpeg subdir without the "external/FFmpeg" prefix. 
Using git subtree would not make the mythtv repo rely on the ffmpeg 
repo. It would just make them share some sha-1 hashes in a useful way.

Before git subtree existed, it was possible to have a local repo for one 
project include remotes from an entirely different project. Without git 
subtree it wasn't useful to do so because if you checked out on a branch 
from one project all the files from the other project would disappear. 
git subtree is just a useful way of using remotes from different 
projects in the case that one project represents the files of a 
subdirectory of another project.