diff: don't use pathname-based diff drivers for symlinks

When we're diffing symlinks, we consider the contents to be the pathname that the symlink points to. When a user sets up a userdiff driver like "*.pdf diff=pdf", their "diff.pdf.*" config generally tells us what to do with the content of pdf files. With the current code, we will actually process a symlink like "link.pdf" using a configured pdf driver, meaning we are using contents which consist of a pathname with configuration that is expecting contents that consist of an actual pdf file. The most noticeable example of this would have been textconv; however, it was already protected in its own textconv-specific code path. We can still see the breakage with something like "diff.*.binary", though. You could also see it with diff.*.funcname, though it is a bit harder to trigger accidentally there. This patch adds a check for S_ISREG lower in the callstack than the textconv-specific check, which should block use of any userdiff config for non-regular files. We can drop the check in the textconv code, which is now redundant. Signed-off-by: Jeff King <peff@peff.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>