scripts: get_source_date_epoch.sh: fix mercurial support, add mtime fallback

Add a fallback case to get_source_date_epoch.sh which reports the modification
time of the script itself in case there is no SCM information available, e.g.
when downloading .tar.gz or .zip tarballs produced by Github.

Also fix the mercurial case while we're at it.

Signed-off-by: Jo-Philipp Wich <jo@mein.io>

scripts/get_source_date_epoch.sh

@@ -17,9 +17,14 @@ try_git() {
 
 try_hg() {
 	[ -d .hg ] || return 1
-	SOURCE_DATE_EPOCH=""
+	SOURCE_DATE_EPOCH="$(hg log --template '{date}' -l 1 | cut -d. -f1)"
 	[ -n "$SOURCE_DATE_EPOCH" ]
 }
 
-try_version || try_git || try_hg || SOURCE_DATE_EPOCH=""
+try_mtime() {
+	perl -e 'print((stat $ARGV[0])[9])' "$0"
+	[ -n "$SOURCE_DATE_EPOCH" ]
+}
+
+try_version || try_git || try_hg || try_mtime || SOURCE_DATE_EPOCH=""
 echo "$SOURCE_DATE_EPOCH"