[PATCH v2] doc-diff: don't `cd_to_toplevel` before calling `usage`

["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. Granted, all of these `usage` calls are in error paths, so
we're about to exit anyway, but the user experience of something like
`(cd Documentation && ./doc-diff)` could be a bit better than
"./doc-diff: not found".

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. Move it back to where it was, and
teach the "--clean" code to cd on its own. This way, we only cd once
we've verified the arguments.

A more general fix would 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), so let's just fix this user
instead.

Signed-off-by: Martin Ågren <martin.agren@gmail.com>
---

 > Punting and extending the commit message like that sounds reasonable.

 So here's a v2 doing exactly that.

 Documentation/doc-diff | 8 +++++---
 1 file changed, 5 insertions(+), 3 deletions(-)

diff --git a/Documentation/doc-diff b/Documentation/doc-diff
index dfd9418778..f820febf8f 100755
--- a/Documentation/doc-diff
+++ b/Documentation/doc-diff
@@ -39,12 +39,11 @@ do
 	shift
 done
 
-cd_to_toplevel
-tmp=Documentation/tmp-doc-diff
-
 if test -n "$clean"
 then
 	test $# -eq 0 || usage
+	cd_to_toplevel
+	tmp=Documentation/tmp-doc-diff
 	git worktree remove --force "$tmp/worktree" 2>/dev/null
 	rm -rf "$tmp"
 	exit 0
@@ -66,6 +65,9 @@ to=$1; shift
 from_oid=$(git rev-parse --verify "$from") || exit 1
 to_oid=$(git rev-parse --verify "$to") || exit 1
 
+cd_to_toplevel
+tmp=Documentation/tmp-doc-diff
+
 if test -n "$force"
 then
 	rm -rf "$tmp"
-- 
2.20.1.309.g16a465bc01