No option to ignore filemode if diff'ing not in a repository?

There is a workaround: Show git diff, ignoring file permission changes? (Zed notes that git diff -G. does the trick). Note: the dot (period) in -G. is the key here. A mode-only diff has no diff text, so . fails to match; all other diffs do have diff text, even if it's a pure delete, so . matches something, and the file gets displayed. But this is just a workaround, not a true fix.

To expand a bit on my comment above: there is indeed no way to do exactly what you wanted (other than temporarily chmod-ing the files, or copying them and chmod-ing the copies).

When you run git diff --no-index (or any diff in which --no-index is implied), Git calls lstat on each file to find out whether it's a regular file or directory and what its mode is. This call is made from diff-no-index.c around line 43, whose last few lines include:

else
        *mode = st.st_mode;

This assumes that the OS, whatever it is, produces consistent st_mode executable bits, even if they cannot be trusted.

To make this obey the core.filemode setting, the last line might need to read, e.g.:

        *mode = !S_IFREG(st.st_mode) || trust_executable_bit ?
            st.st_mode : st.st_mode & ~0111;

for instance. This could perhaps be simplified: I don't think Git cares about the x bits on directories so just always masking away 0111 might suffice, when trust_executable_bit is false.