we now use -P for --prefix and -p for --progress. previously, the
result of -p depended on the command: some were using it for
--progress, some for --prefix. this was confusing and was making it
impossible to both --progress and --prefix with on-letter options
--progress is likely used more often and interactively, so it get the
keystroke shortcut (lower "-p")
--prefix is used more rarely / in scripts, but important/dangerous for
prune, so it get the extra keystroke (higher "-P")
If somebody used -p someprefix and does not fix that to -P, it will
result in "no archive specified" or "unrecognized argument". So it
will neither cause pruning to remove wrong data nor go unnoticed.
Closes: #563
22efee3d2e