doc-diff: don't `cd_to_toplevel`

`usage` tries to call $0, which might very well be "./doc-diff", so if
we `cd_to_toplevel` before calling `usage`, we'll end with an error to
the effect of "./doc-diff: not found" rather than a friendly `doc-diff
-h` output. This regressed in ad51743007 ("doc-diff: add --clean mode to
remove temporary working gunk", 2018-08-31) where we moved the call to
`cd_to_toplevel` to much earlier.

A general fix might be to teach git-sh-setup to save away the absolute
path for $0 and then use that, instead. I'm not aware of any portable
way of doing that, see, e.g., d2addc3b96 ("t7800: readlink may not be
available", 2016-05-31).

An early version of this patch moved `cd_to_toplevel` back to where it
was before ad51743007 and taught the "--clean" code to cd on its own.
But let's try instead to get rid of the cd-ing entirely. We don't really
need it and we can work with absolute paths instead. There's just one
use of $PWD that we need to adjust by simply dropping it.

Suggested-by: Jeff King <peff@peff.net>
Signed-off-by: Martin Ågren <martin.agren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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