apply: symbolic links lack a "trustable executable bit"

apply: symbolic links lack a "trustable executable bit"

When 0482c32c334b (apply: ignore working tree filemode when
!core.filemode, 2023-12-26) fixed `git apply` to stop warning about
executable files, it inadvertently changed the code flow also for
symbolic links and directories.

Let's narrow the scope of the special `!trust_executable_git` code path
to apply only to regular files.

This is needed to let t4115.5(symlink escape when creating new files)
pass on Windows when symbolic link support is enabled in the MSYS2
runtime.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>