cli: add option to list only conflicted branches

[Kintaro]

As requested in #1471, I added a new flag for jj branch list to only show branches that are conflicted.

Adds a unit test to check for listing only conflicted branches and regenerates the cli output to incorporate the new flag.

Closes #1471

reformat

Checklist

If applicable:

• I have updated CHANGELOG.md
• I have updated the documentation (README.md, docs/, demos/)
• I have updated the config schema (cli/src/config-schema.json)
• I have added tests to cover my changes

[yuja]

lgtm, thanks!

[Kintaro]

I have updated it, but had to change the way I was retrieving the output, otherwise jj_cmd_success complained due to

stdout=
bar: kkmpptxz 06a973bc (empty) b
foo (conflicted):
  + rlvkpnrz d8d5f980 (empty) a
  + kkmpptxz 06a973bc (empty) b

stderr="Concurrent modification detected, resolving automatically.\n"

I hope that's ok. If there's a better way how this should be handled in jj, I will gladly change it.

[yuja]

I have updated it, but had to change the way I was retrieving the output, otherwise jj_cmd_success complained due to

The common pattern is let (stdout, stderr) = jj_cmd_ok(..). You can also insert some command (like jj status) before get_branch_output(). The diverged operations will be merged there.

[Kintaro]

Oh right, forgot about the internal operations log. I have added a jj status call and reverted the branch list --all and branch list --conflicted to use jj_cmd_success.