[feature] option to use hash attached to the relative path instead of the version field

[andry81]

Git already does store submodule commit hash in a commit.

Example: andry81-tests/gil-test1-A@f5f753f

Which means if the yaml file has the version field then, it actually by default overrides a submodule branch commit to be checkouted:

repositories:

  _externals/lib1:
    type:         git
    url:          https://github.com/user/lib1
    version:      master

The _externals/lib1 is already committed here with the external repo hash (the url is not known internally).

I suggest to add an option to import command to use versions recorded for all submodules:

>vcs import --use-internal-version < .externals

This will checkout versions has been committed the last time, not the versions declared in the .externals yaml file.