t0040: declare non-tab indentation to be okay in this script

By necessity, this script needs to verify that certain Git output
matches expectations, including text indented with spaces instead of
tabs.

Most recently, such a check was introduced in 448abbb (short help:
allow multi-line opthelp, 2023-07-18) which is reported by `git show
--check` as having whitespace issues.

Let's not complain about this because it is intentional.

Signed-off-by: Johannes Schindelin <[email protected]>

Author: dscho

t/.gitattributes

@@ -22,3 +22,4 @@ t[0-9][0-9][0-9][0-9]/* -whitespace
 /t7500/* eol=lf
 /t8005/*.txt eol=lf
 /t9*/*.dump eol=lf
+/t0040*.sh whitespace=-indent-with-non-tab