pkg-merge-upstream-to-downstreams: Fix targeting stable branches

When targeting development/preview releases the script to merge changes
from upstream works fine, but when aiming at stable releases the part
that tries to detect which branch to target (`-security` vs. `-updates`)
fails to find the refs it looks for because it searches the local refs
instead of the remote ones.

Point it to the refs under the `origin` remote to make it happy.

Signed-off-by: Emanuele Aina <emanuele.aina@collabora.com>

apertis-package-source-builder/overlay/usr/bin/apertis-pkg-merge-upstream-to-downstreams

@@ -17,10 +17,10 @@ def parse_ref(ref: str) -> str:
     return git('rev-parse', '-q', '--verify', ref + '^{commit}', _ok_code=[0, 1]).strip('\n')
 
 def ensure_branch(name: str, fallbacks: list) -> str:
-    if parse_ref(name):
+    if parse_ref(f'origin/{name}'):
         return name
     for b in fallbacks:
-        commit = parse_ref(b)
+        commit = parse_ref(f'origin/{b}')
         if commit:
             print(f'Setting branch {name} to point to {b} ({commit:.7})')
             git('update-ref', f'refs/heads/{name}', commit)