hooks/post-merge: Set file timestamps for updates

Partial updates require that source files have appropriate timestamps,
which Git does not set in any special way by default. Thus, before
attempting to update files in-place, we set source timestamps based on
last commit time, which is the effective update time in any case.

hooks/post-merge

@@ -7,6 +7,7 @@
 # The script assumes write access to host directories, and a CoreOS host. Don't run on other systems!
 
 set -euo pipefail
+shopt -s globstar
 
 # Global configuration variables.
 TEMP_CONFIG_PATH="$(git -C "$(dirname "$0")" rev-parse --show-toplevel)"
@@ -25,10 +26,17 @@ function sync-coreos-config() {
         fi
 
         # Remove files that only exist in local configuration.
-        for n in $(comm -23 <(cd "$path"; find . | sort) <(cd "$dir"; find . | sort)); do
-            rm --verbose --recursive --force "$(realpath --quiet "$path/$n")"
+        for f in $(comm -23 <(cd "$path"; find . | sort) <(cd "$dir"; find . | sort)); do
+            rm --verbose --recursive --force "$(realpath --quiet "$path/$f")"
         done
 
+        # Update timestamp for temporary file to match last commit time, in order to ensure
+        # correct partial updates.
+        for f in "$dir/"**; do
+            touch --date="$(git -C "$TEMP_CONFIG_PATH" log -n 1 --pretty=format:%cd --date=iso --date-order -- "$f")" -- "$f"
+        done
+
+        # Copy files from temporary directory to host configuration directory.
         cp --verbose --recursive --update --target-directory "$path" "$dir"/*
         echo "done."
     done
@@ -36,8 +44,8 @@ function sync-coreos-config() {
 
 # Synchronize systemd service files from CoreOS home-server configuration.
 function sync-systemd-services() {
-    local dest
     local result=1
+    local dest
 
     # Copy service files if newer than destination.
     for src in "$HOST_CONFIG_PATH"/*/systemd/*; do