When importing a tree from a special remote, often content identifiers need
to be written for all files that it finds. Currently, the timestamp varies
a little bit while writing these. If the same timestamp were used for all
the logs, they would delta-compress better.

This probably extends well past content identifiers, but they would be a
probably relatively easy place to do it since they're all written in the
same place.

As a bonus, since borg uses the same content identifiers for all keys
(""), implementing this would make the content of the logs all identical,
and so avoid any overhead entirely. Well.. perhaps that's my actual
motivation. ;-) --[[Joey]]

> [[done]], though reuseVectorClockWhile could be used in other places
> perhaps. --[[Joey]]
