[PATCH v3] doc-diff: don't `cd_to_toplevel`

["Martin Ågren" <martin.agren@gmail.com>]

`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>
---
 Thanks Peff for the suggestions. Trying not to cd at all seems sane to
 me. That it allows `./Documentation/doc-diff` is a bonus, I guess, but
 you're right that it's probably nothing anyone will use.

 I've verified that diffs produced by `./Documentation/doc-diff foo bar`
 and `./doc-diff foo bar` are identical, FWIW.

 Martin

 Documentation/doc-diff | 5 ++---
 1 file changed, 2 insertions(+), 3 deletions(-)

diff --git a/Documentation/doc-diff b/Documentation/doc-diff
index dfd9418778..32c83dd26f 100755
--- a/Documentation/doc-diff
+++ b/Documentation/doc-diff
@@ -39,8 +39,7 @@ do
 	shift
 done
 
-cd_to_toplevel
-tmp=Documentation/tmp-doc-diff
+tmp="$(git rev-parse --show-toplevel)/Documentation/tmp-doc-diff" || exit 1
 
 if test -n "$clean"
 then
@@ -109,7 +108,7 @@ render_tree () {
 		make -j$parallel -C "$tmp/worktree" \
 			GIT_VERSION=omitted \
 			SOURCE_DATE_EPOCH=0 \
-			DESTDIR="$PWD/$tmp/installed/$1+" \
+			DESTDIR="$tmp/installed/$1+" \
 			install-man &&
 		mv "$tmp/installed/$1+" "$tmp/installed/$1"
 	fi &&
-- 
2.20.1.390.gb5101f9297