Try to avoid building the snaps from branches not having snap-related changes,

except for merges (when HEAD is the v0.6 tip).

Author: Lee Miller

.buildbot/snap/build.sh

@@ -1,5 +1,12 @@
 #!/bin/bash
 
+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)"
+SNAP_DIFF="$(git diff upstream/v0.6 -- packages/snap .buildbot/snap)"
+
+[ -z "${SNAP_DIFF}" ] && [ $HEAD != $UPSTREAM ] && exit 0
+
 pushd packages && snapcraft || exit 1
 
 popd