Use complete Debian revision number as build number

Use complete Debian revision number as build number, instead of
just the numeric part. Also, re-implement scripts/time-stamper.bash
without direct calls to dpkg-parsechangelog.

debian/patches/31-improve-hurd-and-kfreebsd-support.patch

@@ -75,7 +75,7 @@ index 60c55f9..f30f5d3 100644
  
  
 diff --git a/closure.c b/closure.c
-index 632c103..7c8b3bd 100644
+index f408f7a..f4b1893 100644
 --- a/closure.c
 +++ b/closure.c
 @@ -50,7 +50,8 @@ static void get_base_dirs()