chlog: add -f/--file to target a specific changelog

The changelog path was hardcoded to debian/changelog in the series
detection, the entry generation and the editor invocation. Packages
keeping the changelog elsewhere (e.g. a staging copy) could not use
chlog on it.

Add -f/--file defaulting to debian/changelog, threaded through all
three uses. The library side was already parameterized on the file,
so no changelog.rs behavior changes beyond a test pinning the
non-default path.

README.md

@@ -112,6 +112,9 @@ pkh put --ppa user/hello_xxx
 git push xxx user-fork
 ```
 
+`pkh chlog` edits `debian/changelog` by default; point it at another file
+with `--file <path>` (`-f`).
+
 `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