REL: use git commit date for version metadata

Use commit time stamp as in git-archive and versioneer.

Author: pavoljuhas

site_scons/gitarchive.cfg

@@ -1,3 +1,3 @@
 commit = $Format:%H$
-date = $Format:%ai$
+date = $Format:%ci$
 refnames = $Format:%D$

site_scons/libobjcrystbuildutils.py

@@ -34,7 +34,7 @@ def gitinfo():
     if proc.wait():
         _cached_gitinfo = {}
         return gitinfo()
-    proc = Popen(['git', 'log', '-1', '--format=%H %ai'], **kw)
+    proc = Popen(['git', 'log', '-1', '--format=%H %ci'], **kw)
     glog = proc.stdout.read()
     words = desc.strip().split('-')
     vtag = words[0].lstrip('v')