completion: generate fish completions, remove hand-written ones

[ThomasWaldmann]

Fixes #9989.

borg completion fish now generates fish shell completions automatically, the same way bash/zsh completions are generated. borg brings its own fish backend rather than using shtab's (which appeared in shtab 1.9.0): shtab's fish backend bases its conditions on __fish_seen_subcommand_from (which can misfire when positional values collide with command names) and is not positional-aware. borg's generator walks the parser tree and emits fish complete commands, plus a preamble with fish versions of the same dynamic completion functions the bash/zsh scripts have (archive names, aid: archive IDs with metadata descriptions, tags, sort keys, files cache modes, compression specs, chunker params, relative time markers, timestamps, file sizes, help topics).

Details:

• shtab >= 1.9.2 is now required (older versions generate broken zsh completions, see zsh: shlex.quote() in help breaks generated script — any apostrophe corrupts quoting (regression in 1.8.2) tqdm/shtab#224, or have no fish support), so shtab.SUPPORTED_SHELLS already contains "fish" and is used as-is for the SHELL choices; borg completion fish dispatches to borg's own backend.
• The generated script parses the command line to track the current (sub)command path (e.g. key export) and the number of positional arguments already given, so completions are position-aware — e.g. borg diff offers archive names for both the first and second positional, and file completion still works for PATH arguments after them.
• Common options (identical in every context) are emitted once without a condition; everything else is guarded by a per-command condition.
• Option strings that take a value are tracked so their values are not mistaken for subcommands/positionals. --keep is a flag in key change-location but takes a value in prune; it is treated as value-taking (the lesser evil).
• The hand-written, unmaintained scripts/shell_completions/fish/borg.fish and its test are removed.
• New tests cover script size/syntax (fish --no-execute) and, via fish's complete -C, subcommand/option/value completion, sort-key dedup, files-cache exclusivity, and archive name / aid: completion against a real repo (running the borg under test via a small PATH shim). CI already installs fish for completion tests.
• Verified locally against shtab 1.9.2: bash/zsh/tcsh/fish generation all work, full completion test suite passes.

Upstream shtab work resulting from this evaluation (issues filed for each problem found in shtab's fish backend, plus fix PRs):

• no file completion / shtab.FILE options mis-complete: fish: no file completion; options marked shtab.FILE complete the wrong candidates tqdm/shtab#227
• no subcommand completion after a global option with a value: fish: no subcommand completion after a global option that takes a value tqdm/shtab#228
• completions vanish when a value equals a subcommand name: fish: completions vanish when an argument value equals a subcommand name tqdm/shtab#229
• positional argument order ignored: fish: positional argument order ignored (earlier positionals' candidates re-offered at later slots) tqdm/shtab#230
• %(default)s unexpanded, subcommand descriptions missing with help=-only: fish: %(default)s not expanded in descriptions; subcommands lack descriptions when only help= is set tqdm/shtab#231
• fix PRs: fish: token-aware conditions, positional slots, file completion tqdm/shtab#232 (token-aware conditions, positional slots, file completion) and fish: expand %(default)s etc., fall back to help for subcommand descriptions tqdm/shtab#233 (description handling), both CI-green

For now, borg ships its own fish backend so this doesn't have to wait on upstream. Once the shtab fixes are merged and released, we will adapt borg to use shtab's fish backend (like it already does for bash/zsh) and remove any then-duplicate fish code here.

🤖 Generated with Claude Code

[codecov]

Codecov Report

❌ Patch coverage is 96.39640% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 85.94%. Comparing base (a293a67) to head (593b1e8).
⚠️ Report is 1 commits behind head on master.
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
src/borg/archiver/completion_cmd.py	96.39%	2 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #9990      +/-   ##
==========================================
+ Coverage   85.88%   85.94%   +0.06%     
==========================================
  Files          95       95              
  Lines       17088    17187      +99     
  Branches     2616     2643      +27     
==========================================
+ Hits        14676    14772      +96     
- Misses       1674     1675       +1     
- Partials      738      740       +2     

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