completions: rejoin bash words split at COMP_WORDBREAKS

[nikosavola]

Bash splits the word being completed at COMP_WORDBREAKS characters, which include @, :, and = by default, so dynamic completion of values like remote bookmarks (jj rebase -d master@) received only fragments of the word and returned nothing. Revset ranges (main::) and args containing = (git push --named a=) were affected the same way.

Wrap the generated bash completion function so that it rejoins the split words before handing them to the completer, and rebases the returned candidates on the part of the word that bash will actually replace. This is the approach git-completion.bash takes for its own word break characters; unlike the alternative of editing COMP_WORDBREAKS (and disabling bash's @-hostname completion, which would re-add @ there), it has no effect on the rest of the shell session.

Fixes #7758

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/updated tests to cover my changes
• I fully understand the code that I am submitting (what it does,
  how it works, how it's organized), including any code drafted by an LLM.
• For any prose generated by an LLM, I have proof-read and copy-edited with
  an eye towards deleting anything that is irrelevant, clarifying anything
  that is confusing, and adding details that are relevant. This includes,
  for example, commit descriptions, PR descriptions, and code comments.