perf: don't clobber startrev variable Previously, the "startrev" argument would be ignored due to "startrev = 0" in the benchmark function. This meant that `hg perfrevlog` always started at revision 0. Rename the local variable to "beginrev" so the variable does the right thing.

bundle: add optional 'tagsfnodecache' data to on disk bundle (issue5543) This should help performance when unbundling.

bundle2: move tagsfnodecache generation in a generic function This will help us reusing the logic for `hg bundle`.