scripts: use 'git checkout -f' instead of 'reset --hard'

'checkout -f' more accurately does what is intended. 'reset' changes the
current branch to some hash, 'checkout -f' force-checkouts another branch.

Change-Id: Ic6279ebaf8160bceb3fa2ab40eff0b888ecd5009

scripts/osmo-deps.sh

@@ -16,5 +16,5 @@ git fetch origin
 # the dep subdir separately:
 osmo-clean-workspace.sh
 
-git reset --hard "$branch"
+git checkout -f "$branch"
 git rev-parse HEAD

scripts/osmo-layer1-headers.sh

@@ -44,4 +44,4 @@ fi
 
 cd layer1-headers
 git fetch origin
-git reset --hard "$version"
+git checkout -f "$version"