Output acid_repo_ref_update errors to stderr
Preserve variable tag set order
Store variable tag map under "map" key
Ignore read's EOF exit code in acid_set.sh
Don't use tag names as descriptions
Avoid unbound variable error with empty sets

Explicitly handle empty sets in acid_set_* functions to prevent the
"unbound variable" error from expanding empty set indexed arrays.

Do this even for functions which expand associative array keys, which
don't produce such errors, in case the functions are rewritten with
indexed arrays, or Bash behavior changes.
Output reference format usage message on errors

Output a message describing target reference format upon receiving
invalid references in acid_repo_ref_update.