download_and_compile.sh: fix the order of operations in _gitUpdate()

_gitUpdate() used to call 'git pull' then 'git checkout'. If the latter
command really did a branch switch, this could leave the local branch
behind its remote counterpart (that was hopefully fetched by the
'git pull'). From now on, we do:

  git fetch origin
  git checkout --force "$branch"
  git pull --rebase

(the --force option was already there before in the form of -f). This
way, we can be sure that the local branch we check out is up-to-date
when the function returns.

Also replace short options by long ones for better readability of the
code, and use Bash's [[ ... ]] conditional construct instead of [ ... ]
for more efficiency---I think.

Note: 'git stash save' is documented as deprecated in favor of
      'git stash push', however the latter appears to have been
      introduced in 2017, therefore I believe it is too early to use it
      in download_and_compile.sh.

download_and_compile.sh

@@ -260,19 +260,22 @@ function _gitUpdate(){
   if [ "$DOWNLOAD" != "y" ]; then
     return
   fi
-  branch="$1"
+
+  local branch="$1"
   set +e
   git diff --exit-code 2>&1 > /dev/null
-  if [ $? != 1 ]; then
+  if [[ $? != 1 ]]; then
     set -e
-    git pull -r
+    git fetch origin
-    git checkout -f "$branch"
+    git checkout --force "$branch"
+    git pull --rebase
   else
     set -e
-    git stash save -u -q
+    git fetch origin
-    git pull -r
+    git stash save --include-untracked --quiet
-    git checkout -f "$branch"
+    git checkout --force "$branch"
-    git stash pop -q
+    git pull --rebase
+    git stash pop --quiet
   fi
 }