git-difftool(1): fix formatting of --symlink description

Signed-off-by: John Keeping <john@keeping.me.uk>
difftool: avoid double slashes in symlink targets

When we add tests for symlinks in "git difftool --dir-diff" it's easier
to check the target path if we don't have to worry about double slashes
separating directories.  Remove the trailing slash (if present) from
$workdir before creating the symlinks in order to avoid this.

Signed-off-by: John Keeping <john@keeping.me.uk>
difftool --dir-diff: symlink all files matching the working tree

Some users like to edit files in their diff tool when using "git
difftool --dir-diff --symlink" to compare against the working tree but
difftool currently only created symlinks when a file contains unstaged
changes.

Change this behaviour so that symlinks are created whenever the
right-hand side of the comparison has the same SHA1 as the file in the
working tree.

Note that textconv filters are handled in the same way as by git-diff
and if a clean filter is not the inverse of its smudge filter we already
get a null SHA1 from "diff --raw" and will symlink the file without
going through the new hash-object based check.

Signed-off-by: John Keeping <john@keeping.me.uk>

---
Changes since v2:
- Use <<-EOF to indent "actual" file content
- Add '|| echo "not a link $f"' to readlink in comparison script