Cap BFS visits in compute_hidden_forks to prevent hangs

[eljaska]

Summary

• Adds a benchmark test (query_bench) that measures evaluate_revset_str, compute_hidden_forks, and get_page across four revset types on any jj repo. The test is #[ignore] and run manually via BENCH_REPO=/path/to/repo cargo test query_bench -- --ignored --nocapture.
• Caps the total BFS visits in compute_hidden_forks at 100,000 to prevent unbounded ancestor traversal when most bookmarks are outside the log revset.

Context

PR #22 replaced the O(B × n) per-bookmark evaluate_revset_expr call with a BFS walk per out-of-log bookmark. The PR assumed "cost is O(depth_to_log) per bookmark, which is tiny in practice since bookmarks are usually a few commits off the log boundary." This holds when the log revset is large (e.g. all() or ::@), since most bookmarks are already in-log or close to it. But with small log revsets like default or mine, nearly all ~4,000 bookmarks of stargate are out-of-log, and each BFS walks thousands of commits before finding a log ancestor — or never finds one. With no bound on total work, this causes an indefinite hang on startup for any repo with many bookmarks and a small default revset.

Before (on master)

revset         eval      forks       page      total  rows
----------------------------------------------------------------------
all()           8ms       36ms       59ms      104ms  50
::@             0ms      484ms       16ms      501ms  50
default         —     TIMEOUT (>30s)
mine            —     never reached

After (with BFS cap at 100k visits)

revset         eval      forks       page      total  rows
----------------------------------------------------------------------
all()           9ms       40ms       66ms      116ms  50
::@             0ms      519ms       18ms      538ms  50
default        34ms     3359ms       20ms     3414ms  35
mine            0ms     3395ms       15ms     3411ms  4

Test plan

• BENCH_REPO=/path/to/repo cargo test query_bench -- --ignored --nocapture completes for all four revsets
• cargo test (non-ignored tests) still passes
• Verified the jjuicy desktop app loads the stargate repo without hanging

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