Be sure not to apply the patch multiple times

There are at least two places in ngx_http_parse.c where the patch can apply,
so be sure to apply it only once. As extra safety, save a backup of that file
and print a diff at the end.
Return to the original directory at the end.

rpm/addPatchToNginxSpec.sh

@@ -6,19 +6,27 @@ set -ex
 # Set the nginx spec file with the httpg patch
 PATCH_NAME="nginx-httpg_no_delegation.patch"
 SPEC_FILE="${HOME}/rpmbuild/SPECS/nginx.spec"
+if grep --extended-regexp --quiet "^Patch.*: $PATCH_NAME" "$SPEC_FILE"; then
+  >&2 echo "The patch $PATCH_NAME is already included in the spec file $SPEC_FILE"
+  exit 0
+fi
 # Copy the patch in the rpmbuild directory
 cp ${CI_PROJECT_DIR}/${PATCH_NAME} ~/rpmbuild/SOURCES/
-cd ~/rpmbuild/SOURCES/
+pushd ~/rpmbuild/SOURCES/
 # Find the highest existing Patch number in the spec file
 LAST_PATCH_NUM=$(grep -oP "^Patch\K[0-9]+" $SPEC_FILE | sort -n | tail -1)
 if [ -z "$LAST_PATCH_NUM" ]; then
   # There are no existing patches: find the highest Source number in the spec file
   LAST_SOURCE_NUM=$(grep -oP "^Source\K[0-9]+" $SPEC_FILE | sort -n | tail -1)
   # Add the patch to the spec file after the last Source
-  sed -i "/^Source${LAST_SOURCE_NUM}/a Patch0: ${PATCH_NAME}" "${SPEC_FILE}"
+  sed -i.backup "/^Source${LAST_SOURCE_NUM}/a Patch0: ${PATCH_NAME}" "${SPEC_FILE}"
 else
   # Add the new patch to the spec file after the last Patch
-  sed -i "/^Patch${LAST_PATCH_NUM}/a Patch$((LAST_PATCH_NUM + 1)): ${PATCH_NAME}" "${SPEC_FILE}"
+  sed -i.backup "/^Patch${LAST_PATCH_NUM}/a Patch$((LAST_PATCH_NUM + 1)): ${PATCH_NAME}" "${SPEC_FILE}"
 fi
 # Add httpg to release
 sed -i '/%define base_release/ s/ngx/httpg/' "${SPEC_FILE}"
+
+diff "${SPEC_FILE}.backup" "${SPEC_FILE}"
+
+popd