FR: add `jj git fetch --quiet`

[jyn514]

Is your feature request related to a problem? Please describe. i want to run jj git fetch programatically (in a shell alias). right now it prints "Nothing changed", or sometimes "Abandoned n commits that are no longer reachable". i would like it to only print if it encounters an error.

Describe the solution you'd like add a new jj git fetch --quiet option.

Describe alternatives you've considered

• redirect stderr to /dev/null. this discards errors in addition to progress messages.
• change jj git fetch to print to stdout instead of stderr. that would work fine, but i don't know why stderr was chosen for info messages to start, so i'm hesitant to change it.

Additional context jj 0.13.0-81b0e3bf3bf6f230740e93532b9a9767e8a4ef6e