upgrade: introduce a _copyrevlog method
This function copies a revlog from the old store to the new one, without
re-adding all deltas manually. This will eventually save a lot of time when some
revlog does not needs any conversions.
Code actually using this will be introduced in later changesets.
$ hg init
$ echo This is file a1 > a
$ hg add a
$ hg commit -m "commit #0"
$ ls
a
$ echo This is file b1 > b
$ hg add b
$ hg commit -m "commit #1"
$ hg co 0
0 files updated, 0 files merged, 1 files removed, 0 files unresolved
B should disappear
$ ls
a