git-debrebase: test suite: gdr-*: use faketime to control dch dates

This means that the dgit- and gdr-generated commits with dates from
changelogs are all adjusted.  This makes the gitk in the gdr tests
look more like a real one would, so we can see if it's pretty.  (It
also makes it easier to debug the tests.)

Signed-off-by: Ian Jackson <ijackson@chiark.greenend.org.uk>