git: vcs import by hash does not work for hashes not in local history

[timor-raiman]

Run the following to reproduce:

git clone [email protected]:timor-raiman/action-ros-ci.git -b vcs-issue && cd action-ros-ci
git fetch origin +refs/pull/3/*:refs/remotes/origin/pr/3/*
export PRMERGE_SHA=`git rev-parse origin/pr/3/merge`
rm -rf new-path/ && mkdir -p new-path/
cat test-vcs-issue.repo | envsubst | vcs import --force --recursive new-path/

Explanation:
vcs does not fetch the git commits corresponding to pull requests (unless it has access to the source branch and we are interested in /head rather than /merge)

Solution:
Given the desired version is a commit hash, vcs should first run git fetch origin <the hash> before attempting to checkout the hash.

[timor-raiman]

@christophebedard, @Nir-Az this needs fixing for ros-tooling/action-ros-ci#816

This is related to ros-tooling/action-ros-ci#812