update README
add err() bash helper
comment some admin/ scripts
comment and clean up `mk-git` command

also add test!
make find-broken-links return a sensible exit code
fix some broken links in log/
make `run-tests` verify no broken links exist
add mistakenly dropped `git-links` helper
add test of find-broken-links
factor test helper code out to test/.helpers-rc
add test of fixing broken symlinks
remove “-s” option on mk-git

wasn’t really doing anything now that I relaxed assumptions about 
whether the command being aliased already exists or not
add file-level comments to everything.

fixes #20
travis fixes
remove redundant log path line
print commands in run-tests
remove redundant test setup line
make prompts deterministic; fix travis flakiness
simpler ‘copy-diffs’ implementation using rsync
copy-diffs test