gitk: Fix bug in parsing multiple revision arguments

gitk: Fix bug in parsing multiple revision arguments

If the user specified multiple revisions arguments on the command
line or for a view, we were passing the whole list of arguments to
git rev-parse as a single argument, and thus git rev-parse didn't
interpret it as revisions.  This fixes it by adding an eval so the
arguments get passed to git rev-parse as separate arguments.

Signed-off-by: Paul Mackerras <paulus@samba.org>