Add the alpha siffix for non-merge appimage builds

.buildbot/appimage/build.sh

@@ -4,7 +4,11 @@ export APPIMAGE_EXTRACT_AND_RUN=1
 BUILDER=appimage-builder-x86_64.AppImage
 RECIPE=packages/AppImage/AppImageBuilder.yml
 
+git remote add -f upstream https://github.com/Bitmessage/PyBitmessage.git
+HEAD="$(git rev-parse HEAD)"
+UPSTREAM="$(git merge-base --fork-point upstream/v0.6)"
 export APP_VERSION=$(git describe --tags | cut -d- -f1,3 | tr -d v)
+[ $HEAD != $UPSTREAM ] && APP_VERSION="${APP_VERSION}-alpha"
 
 function set_sourceline {
     if [ ${ARCH} == amd64 ]; then