update: Handle Git-tracked import-ed files

[Baranowski]

After #2889, dvc import can also import files that are tracked by Git but not DVC. When the user tries to dvc update them, the operation fails. See also #2959

[pared]

reproduction script

#!/bin/bash

rm -rf repo remote_repo storage
mkdir repo remote_repo storage

set -e
set -x
main_dir=$(pwd)
pushd remote_repo

git init --quiet && dvc init -q
echo content >> git_file
git add git_file
git commit -am "init"

git add git_file

popd
pushd repo

git init --quiet && dvc init -q
dvc import -q $main_dir/remote_repo git_file
dvc update git_file

Do I get it right, that we want to extend update functionality? So far target could be only *.dvc file.
If we want to extend the functionality, we probably should add the ability to update output by its name. That should probably be solved in #2458 but better keep info on that also in this issue.
[EDIT]
It won't be soleved by #2458, though we should probably solve it only after #2458 is solved, as related logic will probably be implemented there.

[fabiosantoscode]

@pared I think this issue is about imported files which are tracked in a remote non-DVC git repository.

If you change the last line of your repro script to dvc update git_file.dvc you get the right repro:

+ dvc update git_file.dvc
ERROR: failed to update 'git_file.dvc'. - config file error: Section 'remote "auto-generated-upstream"' a
ready exists. Use `-f|--force` to overwrite section with new value.

[fabiosantoscode]

My bad, you also have to not run dvc init in remote_repo

[fabiosantoscode]

I'm on this one. It's a tiny tiny fix.