blame,cat-file: Demonstrate --textconv is wrongly running converter on symlinks

blame,cat-file: Demonstrate --textconv is wrongly running converter on symlinks

git blame --textconv is wrongly calling the textconv filter on
symlinks: symlinks are stored as blobs whose content is the target of
the link, and blame calls the textconv filter on a temporary file
filled-in with the content of this blob.

For example:

    $ git blame -C -C regular-file.pdf
    Error: May not be a PDF file (continuing anyway)
    Error: PDF file is damaged - attempting to reconstruct xref table...
    Error: Couldn't find trailer dictionary
    Error: Couldn't read xref table
    Warning: program returned non-zero exit code #1
    fatal: unable to read files to diff

That errors come from pdftotext run on symlink.pdf being extracted to
/tmp/ with one-line plain-text content pointing to link destination.

So several failures are demonstrated here:

  - git cat-file --textconv :symlink.bin    # also HEAD:symlink.bin
  - git blame --textconv symlink.bin
  - git blame -C -C --textconv regular-file # but also looks on symlink.bin

At present they all fail with something like.

    E: /tmp/j3ELEs_symlink.bin is not "binary" file

NOTE: git diff doesn't try to textconv the pathnames, it runs the
textual diff without textconv, which is the expected behavior.

Signed-off-by: Kirill Smelkov <kirr@landau.phys.spbu.ru>
Reviewed-by: Matthieu Moy <Matthieu.Moy@grenoble-inp.fr>
Signed-off-by: Junio C Hamano <gitster@pobox.com>