Check that executable file is in-tree before failing tidy check
I silenced stdout and stderr for ls-files, not sure if that's appropriate (is `make tidy` intended to give debugging information)? Otherwise it prints each file it find to stdout/stderr, which currently prints nothing (only executable files are checked).
I have not done major testing regarding the behavior of ls-files when the file is ignored, but judging by the man page everything should be fine.
I've duplicated the code which makes the path git-friendly from the `Cargo.lock` checking code; I can extract that into a common helper if wanted (it's only two lines).
Fixes#35689.
57a1f684cd