Add fetch into Git "up" function

In [a previous commit], we removed the fetching of the "origin" remote
from the "up" Git function. With this change, developers had to fetch
manually before rebasing their branch. We added the fetch back into the
"up" Git function.

[a previous commit]: f252ba46d1 (diff-b973d5fb5d66981ace5c069881095355)

bin/git-up

@@ -2,6 +2,8 @@
 
 set -e
 
+git fetch origin
+
 if git symbolic-ref --short refs/remotes/origin/HEAD >/dev/null; then
   git rebase "$(git symbolic-ref --short refs/remotes/origin/HEAD | sed 's@^origin/@@')" "$@"
 else