completion: no directory completion for repository URLs, fixes #10460

[ThomasWaldmann]

Fixes #10460.

-r/--repo and --other-repo completed local directories even when the value already started with a URL scheme:

• fish: borg repo-create --repo=ssh://<TAB> offered ssh://etc/, ssh://usr/, ... and --repo rclone:<TAB> offered rclone:bin/, ... (fish completes the part after the : as a path).
• tcsh: attempted directory completion and printed ssh:// not found.
• bash and zsh showed no candidates, but only because the typed text did not literally match a directory name.

Now, once the value starts with a URL scheme other than file: (the ones Location parses as a URL: ssh, sftp, http, https, s3, b2, rclone), there is no completion in any shell. Local paths (and file:// in fish) still complete directories as before.

• bash, zsh, fish: new _borg_complete_repo_dirs helper: nothing for a URL, else the previous directory completion (fish also handles the attached -rURL form).
• tcsh: can not make a rule depend on the current word, so a leading rule 'c/{--repo=,-r=,--other-repo=,}{ssh,sftp,...}:/n/' completes nothing for a URL; tcsh uses the first matching rule, so it wins over the directory rules. Side effect: any word starting with such a scheme gets no completion in tcsh.

Tested interactively in all four shells, plus new regression tests per shell (they fail without the fix).

Not included: completing hostnames from /etc/hosts / ~/.ssh/config for ssh:// (could be a follow-up).

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 88.73%. Comparing base (b03b096) to head (fd71c9e).
⚠️ Report is 3 commits behind head on master.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #10472      +/-   ##
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+ Coverage   88.71%   88.73%   +0.01%     
==========================================
  Files         103      103              
  Lines       19392    19399       +7     
  Branches     3024     3025       +1     
==========================================
+ Hits        17204    17214      +10     
+ Misses       1518     1515       -3     
  Partials      670      670              

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