support git files as input to computations

support git files as input to computations

Using GIT keys, like are used when exporting git files to special
remotes. Except here the GIT key refers to a file checked into the git
repo.

Note that, since the compute remote uses catObject to get the content,
a symlink that is checked into git does not get followed. This is important
for security, because following a symlink and adding the content to the
repo as an annex object would allow exfiltrating content from outside
the repository.

Instead, the behavior with a symlink is to run the computation on the
symlink target. This may turn out to be confusing, and it might be worth
addcomputed checking if the file in git is a symlink and erroring out.
Or it could follow symlinks as long as the destination is a file in the
repisitory.