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.

upgrade: rename `_copyrevlogs` to `_clonerevlogs` The underlying revlog method is named `clone`, keeping the naming consistent seems clearer. This is motivated to clarify the difference with an (upcoming) function that simply copy revlog files as is.

upgrade: walk the source store file only once I don't expect this to have a significant performance impact, but it seems simpler and saner to do the operation only once and to keep the result around.

upgrade: always use full text if "full-add" mode is enable We should not be using a delta since the goal is to perform a full addition from scratch in all cases. Without this patch, `hg debugupgraderepo --optimize re-delta-fulladd --run` can crash.

byteify-strings: fix misalignment with multi-line parenthesis This improves the current fix to also take into account cases where the last line ended on the opening `(`, `[` or `{` and adds a regression test.

byteify-strings: add test for byteify-strings.py This tests the basic features expected from this script, some cases may not be covered yet. A future patch will demonstrate an issue with multi-line `(`, `[` and `{` alignment and propose a fix.

merge with stable