Feature request: cp/mv: implement advcpmv patches

While this doesn't making cp and mv 100% compatible with GNU variants, this progress bar functionality is very useful

Source: https://github.com/jarun/advcpmv

I agree

I could see adding a -progress option, or some such. So by default they worked like the coreutils versions, but would have the option of showing progress.

Of course, I don't know that they collect all the info required up front to actually display the progress properly... seem like that would slow things down a bit, at least in some cases.

Implemented in #230.

Both tools take a --progress option, off by default, which draws a bar on
standard error while the bytes move:

$ cp --progress -r src dst
[=============                           ] 33%
[===============================         ] 77%
[========================================] 100%

The bar is forty columns wide with a percentage, drawn with a carriage return so
each update overwrites the last. It is on stderr because that is where advcpmv
draws, and because it keeps the tool's own stdout clean.

The size of the source is stat'd before the first byte moves. That is what the bar
needs in order to show a percentage, and it is why --progress is slower than a
plain copy. The bar lives in common/progress.v so both tools share it.

Two things worth knowing for anyone using it:

A long option with no short form is spelled with two dashes. cp -progress
leaves it as a free argument and the copy fails; cp --progress works.
mv's directory move removes the source tree after copying, file by file,
because rmdir only takes an empty directory.

cp and mv had no test files before this, so both are new.