meson: Fix update-man-rules when the build dir is not a subdir of the project dir

meson: Fix update-man-rules when the build dir is not a subdir of the project dir

Until now, update-man-rules assumed that the build directory was
a subdirectory of the project directory. When using mkosi, this is
not the case. We use find instead of git ls-files because git ls-files
does not seem to support outputting absolute paths.

Also, this makes update-man-rules a bit more user-friendly as new manpages
don't have to be added to the git staging area before they are processed by
update-man-rules.py.