largefiles: use common function to build content of .hg_archival.txt
This fixes the missing "changessincelatesttag" field introduced by
242d11819c6c.
archive: extract metadata() closure to module-level function
This function will be reused in largefiles.
templatekw: have {manifest} use ctx.manifestnode() for consistency
changeset_printer was updated at
f57640bf10d4 to not access changeset by
index.
subrepo: inline reporelpath into abstractsubrepo._relpath to centralize logic
"reporelpath()" is referred only from "abstractsubrepo._relpath()".