[PATCH] Prevent git-rev-list --merge-order producing duplicates in the output

[PATCH] Prevent git-rev-list --merge-order producing duplicates in the output

If a is reachable from b, then git-rev-list --merge-order b a would
produce a duplicate output of b.

This causes a problem for an upcoming version of gitk since it
breaks the --merge-order ordering invariant.

This patch fixes the problem for the --merge-order switch. A subsequent
patch will fix the problem for the non --merge-order switch.

Signed-off-by: Jon Seymour <redacted>
Signed-off-by: Linus Torvalds <redacted>