When archiving multiple files, the full length of the calculated
`arcroot` would be removed from the beginning of all file names. If
there was no arcroot, the first two characters of all files would be
removed, so `file.txt` would become `le.txt`.
This patch uses the regular expressions substitution anchored to the
start of the string, so the arcroot is only removed if it is actually
present.
bbe8e1f53b