looponfail output issues w/ looponfailroots

when using xdist "-f, --looponfail" and large number of "looponfailroots" folders is set, output in shell is almost useless because you need to scroll each time something changes.

example output, some content removed:

### LOOPONFAILING ###
unit/test_...
### waiting for changes ###
### Watching:   ...
### Watching:   ...
### Watching:   ...
### Watching:   ...
### Watching:   ...
### Watching:   ...
### Watching:   ...
### Watching:   ...
### Watching:   ...
### Watching:   ...
### Watching:   ...
### Watching:   ...
### Watching:   ...
### Watching:   ...
### Watching:   ...
### Watching:   ...
### Watching:   ...
### Watching:   ...
### Watching:   ...
### Watching:   ...
### Watching:   ...
### Watching:   ...
### Watching:   ...
### Watching:   ...
### Watching:   ...
### Watching:   ...
### Watching:   ...
### Watching:   ...
### Watching:   ...
### Watching:   ...
### Watching:   ...

a somewhat expected response would be to set less roots and rearrange project or that i'm doing something wrong.
here is a simple example why this is irrelevant:
a project using python and javascript can have various workers generating data and logs which all trigger looponfail.

possible solutions:

• allow regex(s) to specify which files/folders should be watched
• display "Watching ..." for looponfailroots only once when "py.test -f" starts

Start by reproducing the shell output with pytest-xdist's -f/--looponfail option and many looponfailroots folders. Compare the proposed filtering and one-time display approaches, then verify that loop-on-failure output remains useful when multiple project workers generate watched files.