mkosi: do a sparse checkout of debian/ubuntu packaging repo

The repository on Salsa includes the full upstream sources, which means
they are duplicated, taking extra space and showing duplicated grep results.
But we only need the debian/ subfolder, so do a sparse clone and checkout.

Author: bluca

mkosi.images/system/mkosi.conf.d/10-debian-ubuntu/mkosi.conf

@@ -7,6 +7,7 @@ Distribution=|ubuntu
 [Content]
 Environment=
         GIT_URL=https://salsa.debian.org/systemd-team/systemd.git
+        GIT_SUBDIR=debian
         GIT_BRANCH=debian/master
         GIT_COMMIT=5b9607385d49c09440e6e3b35c03ceec73162aec
 

mkosi.images/system/mkosi.sync

@@ -13,8 +13,21 @@ if [[ -d "$PKG_SUBDIR/.git" ]] && [[ "$(git -C "$PKG_SUBDIR" rev-parse HEAD)" ==
     exit 0
 fi
 
+# The repository on Salsa has the full upstream sources, so it's a waste of space to
+# redownload and duplicate everything, so do a sparse checkout as we only need the
+# packaging directory anyway
+if [[ -n "${GIT_SUBDIR:-}" ]]; then
+    sparse=(--no-checkout --filter=tree:0)
+else
+    sparse=()
+fi
+
 if [[ ! -e "$PKG_SUBDIR" ]] || [[ -z "$(ls --almost-all "$PKG_SUBDIR")" ]]; then
-    git clone "$GIT_URL" --branch "$GIT_BRANCH" "$PKG_SUBDIR"
+    git clone "$GIT_URL" --branch "$GIT_BRANCH" "${sparse[@]}" "$PKG_SUBDIR"
+    if [[ -n "${GIT_SUBDIR:-}" ]]; then
+        # --no-cone is needed to check out only one top-level directory
+        git -C "$PKG_SUBDIR" sparse-checkout set --no-cone "${GIT_SUBDIR:-}"
+    fi
 else
     git -C "$PKG_SUBDIR" remote set-url origin "$GIT_URL"
     git -C "$PKG_SUBDIR" fetch origin "$GIT_BRANCH"