fetch-distro: use git log --first-parent and update debian commit (#35151)

mkosi.images/build/mkosi.conf.d/debian-ubuntu/mkosi.conf

@@ -9,7 +9,7 @@ Environment=
         GIT_URL=https://salsa.debian.org/systemd-team/systemd.git
         GIT_SUBDIR=debian
         GIT_BRANCH=debian/master
-        GIT_COMMIT=2f288667e0f5eeba19b14d24b621baef2aa413e1
+        GIT_COMMIT=48fabbd5d240a70fce6712b6161f29b40b2fc7de
         PKG_SUBDIR=debian
 
 [Content]

tools/fetch-distro.py

@@ -96,7 +96,7 @@ def update_distro(args, distro: str, config: dict):
         print(f'{distro}: commit {new_commit!s} is still fresh')
         return
 
-    cmd = ['git', '-C', f'pkg/{distro}', 'log', '--graph',
+    cmd = ['git', '-C', f'pkg/{distro}', 'log', '--graph', '--first-parent',
            '--pretty=oneline', '--no-decorate', '--abbrev-commit', '--abbrev=10',
            f'{old_commit}..{new_commit}']
     print(f"+ {shlex.join(cmd)}")