fix-daily-branch: merge instead of reset

Resetting causes discontinuous history.  This tool will handle such
history (because it always resets), but it may cause problems for uses
of the daily branch which this tool does not cover.

Also clean up output a little, to make the messages from the script
easier to identify.

scripts/fix-daily-branch

@@ -8,6 +8,7 @@ set -e
 
 error() { echo "$@" 1>&2; }
 fail() { [ $# -eq 0 ] || error "$@"; exit 1; }
+log() { echo "== $@ =="; }
 
 gitcmd() {
     git "$@" || fail "Failed running git $@"
@@ -35,11 +36,18 @@ drop_cherry_picks_from_daily_branch() {
     local source_branch="${remote}/ubuntu/${release}"
     local daily_branch="ubuntu/daily/${release}"
 
+    log "Checking out and updating daily branch"
     gitcmd fetch ${remote}
-    echo "Backing out all debian/patches/*cpick* from ${daily_branch} branch"
-    gitcmd checkout -B ${daily_branch} ${source_branch}
+    gitcmd checkout ${daily_branch}
+    gitcmd pull ${remote} ${daily_branch}
+    old_daily_branch_head=$(git rev-parse HEAD)
+
+    log "Merging packaging branch (so we can revert patches from it)"
+    gitcmd merge -m "Merge $release packaging branch to daily branch" ${source_branch}
+
     cpick_files=$( find debian/patches -name "*cpick*")
     if [ -n "${cpick_files}" ]; then
+        log "Backing out all debian/patches/*cpick* from ${daily_branch} branch"
         gitcmd log --oneline -- ${cpick_files} | cut -d" " -f1 | xargs git revert
         cat <<EOF
 To fix daily recipe builds perform the following:
@@ -49,8 +57,8 @@ To fix daily recipe builds perform the following:
     git push ${remote} ${daily_branch}
 EOF
     else
-        echo "No fix-daily-branch needed for ${daily_branch}."
-        echo "No debian/patches/*cpick* files present to revert" 
+        log "No debian/patches/*cpick* files present; no reverts needed, undoing merge"
+        gitcmd reset --hard $old_daily_branch_head
     fi
 }