guard against wrong timestamps in git log

For example, my sound repo has in the git-annex branch a commit from
2036, which is followed by one from 2034, in amoung commits from 2013.
Clearly there was a problem with the clock.

Since git log --date-order has a behavior of
"Show no parents before all of its children are shown", the data still
gets processed ok. The future timestamp just prevented displaying data
after that commit. It seems better, when the clock was wrong, to display
a wrong date, and then return to right dates.

It would be nice to filter out the wrong dates from display entirely,
but that seems it would need to buffer the whole output. This command is
too slow to buffer it all before displaying anything, and anyway this
kind of problem is probably rare.

Sponsored-by: Joshua Antonishen on Patreon

Command/Log.hs

@@ -408,6 +408,7 @@ sizeHistoryInfo mu o = do
 		  && (prevoutput /= Just output) = do
 			displayts zone t output
 			return (zone, True, t, Just output)
+		| t < prevt = return (zone, displayedyet, t, Just output)
 		| otherwise = return (zone, displayedyet, prevt, Just output)
 	  where
 		output = intercalate "," (map showsize sizes)