rev-branch-cache: stop truncating cache file
Truncating the file prevent the safe use of mmap. So instead of overwrite the
existing data. If more than 20% of the file is to be overwritten, we rewrite the
whole file instead.
Such whole rewrite is done by replacing the old one with a new one, so mmap of
the old file would be affected.
This prepare a more aggressive use of mmap in later patches.
# So GitLab doesn't think we're using tons of Perl
*.t -linguist-detectable