merge.py: Use git ls-files to get list of downstream PO files

[jeanas]

The previous method could lead to problems if a dependency installed
within venv/ shipped with its own PO files.

[jeanas]

Cf. #1880 (comment).

[JulienPalard]

Wow TIL git does its own glob expansion by default.

Wow that's funny then in bash, as git and bash don't interpret *.po the same way, and as bash leaves the raw *.po when no matches are to be found:

• on python-docs-fr git ls-files *.po only matches 7 files (bash does the glob expansion)
• on python-docs-fr git ls-files '*.po' matches all files (git does the glob expansion)
• on git repo git ls-files *.po matches all files (bash fails to glob expand so it left the literal *.po for git to expand it using its semantics...).

But there's no bash involved in your PR so it's 100% right, git will expand properly every time, nicely done!

[deronnax]

hat down 👏