chore: retry git push sync

During the release it can happen, that two pushes are executed fast one after another.
This could lead to gh action jobs overlapping while trying to push to sourceforge leading
to errors like "remote: error: cannot lock ref 'refs/heads/master': is at XXX but expected
YYY".

.ci/git-repo-sync.sh

@@ -27,7 +27,7 @@ function git_repo_sync() {
     pmd_ci_log_group_start "Git Sync"
         git remote add pmd-sf "${PMD_SF_USER}@git.code.sf.net:/p/pmd/code"
         if [ -n "${PMD_CI_BRANCH}" ]; then
-            git push pmd-sf "${PMD_CI_BRANCH}:${PMD_CI_BRANCH}"
+            retry 5 git push pmd-sf "${PMD_CI_BRANCH}:${PMD_CI_BRANCH}"
             pmd_ci_log_success "Successfully pushed ${PMD_CI_BRANCH} to sourceforge"
         elif [ -n "${PMD_CI_TAG}" ]; then
             git push pmd-sf tag "${PMD_CI_TAG}"
@@ -39,6 +39,43 @@ function git_repo_sync() {
     pmd_ci_log_group_end
 }
 
+
+#
+# From: https://gist.github.com/sj26/88e1c6584397bb7c13bd11108a579746
+#
+# Retry a command up to a specific number of times until it exits successfully,
+# with exponential back off.
+#
+#  $ retry 5 echo Hello
+#  Hello
+#
+#  $ retry 5 false
+#  Retry 1/5 exited 1, retrying in 1 seconds...
+#  Retry 2/5 exited 1, retrying in 2 seconds...
+#  Retry 3/5 exited 1, retrying in 4 seconds...
+#  Retry 4/5 exited 1, retrying in 8 seconds...
+#  Retry 5/5 exited 1, no more retries left.
+#
+function retry {
+  local retries=$1
+  shift
+
+  local count=0
+  until "$@"; do
+    exit=$?
+    wait=$((2 ** $count))
+    count=$(($count + 1))
+    if [ $count -lt $retries ]; then
+      echo "Retry $count/$retries exited $exit, retrying in $wait seconds..."
+      sleep $wait
+    else
+      echo "Retry $count/$retries exited $exit, no more retries left."
+      return $exit
+    fi
+  done
+  return 0
+}
+
 git_repo_sync
 
 exit 0