difftool: avoid double slashes in symlink targets

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 <redacted>
Signed-off-by: Junio C Hamano <redacted>