cmdlib: support statoverride file in root of an overlay.d directory

If an overlay.d directory contains a file called "statoverride" at the
root, pass it to ostree.  This allows overlay files to have permission
bits that aren't tracked by Git.

statoverride files don't support comments.  For convenience, filter out
lines starting with "#".

src/cmdlib.sh

@@ -254,14 +254,26 @@ prepare_build() {
 }
 
 commit_overlay() {
-    local name path respath
+    local name path respath ovrsrc ovrrelsrc
     name=$1
     path=$2
     respath=$(realpath "${path}")
+    # Apply statoverrides from a file in the root of the overlay, which may
+    # or may not exist.  ostree doesn't support comments in statoverride
+    # files, but we do.
+    if [ -e "${respath}/statoverride" ]; then
+        ovrsrc="${respath}/statoverride"
+        ovrrelsrc=/statoverride
+    else
+        ovrsrc=/dev/null
+        ovrrelsrc=
+    fi
     echo -n "Committing ${name}: ${path} ... "
     ostree commit --repo="${tmprepo}" --tree=dir="${respath}" -b "${name}" \
         --owner-uid 0 --owner-gid 0 --no-xattrs --no-bindings --parent=none \
-        --mode-ro-executables --timestamp "${git_timestamp}"
+        --mode-ro-executables --timestamp "${git_timestamp}" \
+        --statoverride <(sed -e '/^#/d' "$ovrsrc") \
+        --skip-list <(echo "$ovrrelsrc")
 }
 
 # Implement support for automatic local overrides: