gitk: Fix direction of symmetric difference in optimized mode

ee66e08 (gitk: Make updates go faster, 2008-05-09) implemented an optimized mode where gitk parses the arguments with rev-parse, and manually reads history in chunks. As mentioned in the commit message, symmetric differences are a problem there: One wrinkle is that we have to turn symmetric diff arguments (of the form a...b) back into symmetric diff form so that --left-right still works, as git rev parse turns a...b into a b ^merge_base(a,b). However, git-rev-parse returns a...b in the swapped order b a ^merge_base(a,b) This has been the case since at least 1f8115b (the state of master at the time of the abovementioned ee66e08; Merge branch 'maint', 2008-05-08). So gitk flipped the sides of symmetric differences whenever it was in optimized mode. Fix this by swapping the sides of the reconstruction code. Signed-off-by: Thomas Rast <trast@student.ethz.ch> Signed-off-by: Paul Mackerras <paulus@samba.org>