tools/docs/checktransupdate.py: add support for scanning directory

tools/docs/checktransupdate.py: add support for scanning directory

Origin script can only accept a file as parameter, this commit enables
it to scan a directory.

Usage example:
./scripts/checktransupdate.py Documentation/translations/zh_CN/dev-tools

And it will output something like:
"""
[1772967203.351603] Documentation/translations/zh_CN/dev-tools/kmemleak.rst
[1772967205.074201] commit 7591c127f3b1 ("kmemleak: iommu/iova: fix transient kmemleak false positive")
[1772967205.074337] 1 commits needs resolving in total

[1772967205.301705] Documentation/translations/zh_CN/dev-tools/index.rst
[1772967206.912395] commit a592a36e4937 ("Documentation: use a source-read extension for the index link boilerplate")
[1772967206.921424] commit 6eac13c87680 ("Documentation: dev-tools: add container.rst page")
[1772967206.930220] commit 8f32441d7a53 ("Documentation: Add documentation for Compiler-Based Context Analysis")
[1772967206.939002] commit 1e9ddbb2cd34 ("docs: Pull LKMM documentation into dev-tools book")
[1772967206.948636] commit d5af79c05e93 ("Documentation: move dev-tools debugging files to process/debugging/")
[1772967206.957562] commit d5dc95836147 ("kbuild: Add Propeller configuration for kernel build")
[1772967206.966255] commit 315ad8780a12 ("kbuild: Add AutoFDO support for Clang build")
[1772967206.966410] 7 commits needs resolving in total
"""

Signed-off-by: Haoyang LIU <tttturtleruss@gmail.com>
[jc: tweaked coding style]
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Message-ID: <20260308111314.27333-1-tttturtleruss@gmail.com>