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Add Jujutsu (jj) support

Parallels the existing Mercurial backend: auto-detects .jj repositories
(takes precedence over .git in colocated layouts so reads go through the
jj working-copy model), translates git-style refs (HEAD → @-, HEAD~N → @
plus N+1 dashes, A..B → --from/--to) to jj revsets, and pipes jj diff
--git output through parseUnifiedDiff. jj emits raw bytes for binary
files instead of the "Binary files … differ" marker, so a pre-parse pass
rewrites such hunks so the existing binary placeholder path kicks in.

--all-files now also supports jj via NewJjDirectoryReader (jj file list);
Mercurial remains unsupported for that mode. Adds jj blame through jj
file annotate with a tab-separated template.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>