The motivating use-case was this `jj signoff` script: https://gist.github.com/thoughtpolice/8f2fd36ae17cd11b8e7bd93a70e31ad6
Which includes lines like this:
```sh
NAME=$(jj config list user.name | awk '{split($0, a, "="); print a[2];}' | tr -d '"')
MAIL=$(jj config list user.email | awk '{split($0, a, "="); print a[2];}' | tr -d '"')
```
There is no reason that we should have to clumsily parse out the config values. This `jj config get` command supports scripting use-cases like this.
Accept an --include-defaults arg to enable including those.
Listing a nonexistent name is no longer an error, but does output a
warning to stderr when no config entries match to explain why there's no
other output.
I don't think need to write non-UTF8 bytes to our config files. If we
ever do (maybe to test that we give the user a reasonable error
message), we add a custom function for that.
Since per-repo config may contain CLI settings, it must be visible to CLI.
Therefore, UserSettings::with_repo() -> RepoSettings isn't used, and its
implementation is nullified by this commit.
#616
