chlog: cap the no-tag history search with --limit

Without a version tag, the boundary of a new entry was found by walking
the whole history and diffing each commit's tree against the changelog
path. On large repositories that per-commit tree diff takes 10+ minutes.

Add --limit (default 500, 0 = unlimited) capping that walk: when no
changelog commit is found within the last N commits, the oldest visited
commit becomes the boundary — the version tag probe is skipped there,
since it could jump the boundary arbitrarily far back and defeat the
cap — and a warning explains the fallback. A found version tag still
takes precedence and is unaffected by the limit.

README.md

@@ -112,6 +112,11 @@ pkh put --ppa user/hello_xxx
 git push xxx user-fork
 ```
 
+`pkh chlog` walks the commit history back to the previous changelog entry.
+Without a version tag, that search is capped at the last 500 commits
+(`--limit <n>`, `0` = unlimited): beyond the cap, the entry simply starts
+at the oldest commit searched, which keeps the run fast on large histories.
+
 ### Incremental builds (build sessions)
 
 Every `pkh deb` build records a **session** under `/var/tmp/pkh/sessions`: