abuild: set SOURCE_DATE_EPOCH to last commit date by default

set datestamps to be used in the built packages to date of commit. This
makes it much easier to have reproducible builds.

abuild.in

@@ -30,11 +30,6 @@ fi
 
 apk_opt_wait="--wait 30"
 
-if [ -z "$SOURCE_DATE_EPOCH" ]; then
-	SOURCE_DATE_EPOCH=$(date -u "+%s")
-fi
-export SOURCE_DATE_EPOCH
-
 umask 022
 
 shell_escape() {
@@ -872,6 +867,11 @@ git_last_commit() {
 	$git log --format=oneline -n 1 "$startdir" | awk '{print $1}'
 }
 
+# date of last commit
+git_last_commit_epoch() {
+	$git log -1 --format=%cd --date=unix -- "$startdir"
+}
+
 get_maintainer() {
 	if [ -z "$maintainer" ]; then
 		maintainer=$(awk -F': ' '/# *Maintainer/ {print $2}' "$APKBUILD")
@@ -2548,6 +2548,14 @@ BUILD_ROOT=
 
 export CC CXX
 
+if [ -z "$SOURCE_DATE_EPOCH" ]; then
+	SOURCE_DATE_EPOCH=$(git_last_commit_epoch)
+fi
+if [ -z "$SOURCE_DATE_EPOCH" ]; then
+	SOURCE_DATE_EPOCH=$(date -u "+%s")
+fi
+export SOURCE_DATE_EPOCH
+
 cd "$startdir" || die
 . "$APKBUILD"