generate more compact git-annex branch for imports

Especially from borg, where the content identifier logs
all end up being the same identical file!

But also, for other imports, the location tracking logs can,
in some cases, be identical files.

Bonus optimisation: Avoid looking up (and parsing when set)
GIT_ANNEX_VECTOR_CLOCK env var every time a log is written to.
Although the lookup does happen at startup even when no
log will be written now.

Command/Forget.hs

@@ -35,7 +35,7 @@ seek = commandAction . start
 
 start :: ForgetOptions -> CommandStart
 start o = starting "forget" ai si $ do
-	c <- liftIO currentVectorClock
+	c <- currentVectorClock
 	let basets = addTransition c ForgetGitHistory noTransitions
 	let ts = if dropDead o
 		then addTransition c ForgetDeadRemotes basets