Fix bare var, use exec so we return the underlying exit code

Author: unixorn

bin/git-track

@@ -4,4 +4,4 @@
 # `origin/$branch-name`.
 
 branch=$(git rev-parse --abbrev-ref HEAD)
-git branch $branch --set-upstream-to origin/$branch
+exec git branch "$branch" --set-upstream-to "origin/$branch"