gitk: Let git-rev-list do the argument list parsing

This is a fix for a problem reported by Jim Radford where an argument
list somewhere overflows on repositories with lots of tags.  In fact
it's now unnecessary to use git-rev-parse since git-rev-list can take
all the arguments that git-rev-parse can.  This is inspired by but not
the same as the solutions suggested by Jim Radford and Linus Torvalds.

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