git-gui: Show ref last update times in revision chooser tooltips

git-gui: Show ref last update times in revision chooser tooltips

If we can we now show the last modification date of a loose ref as
part of the tooltip information shown in the revision picker.  This
gives the user an indication of when was the last time that the ref
was modified locally, and may especially be of interest when looking
at a tracking branch.

If we cannot find the loose ref file than we try to fallback on the
reflog and scan it for the date of the last record.  We don't start
with the reflog however as scanning it backwards from the end is not
an easy thing to do in Tcl.  So I'm being lazy here and just going
through the entire file, line by line.  Since that is less efficient
than a single stat system call, its our fallback strategy.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>