t4150: fix warning printed by awk due to escaped '\@'

t4150: fix warning printed by awk due to escaped '\@'

In 6aec8d38fdd (t: refactor tests depending on Perl to print data,
2025-04-03) we have changed one of the tests in t4150 to use awk(1)
instead of Perl. The test works, but at least gawk(1) prints a warning
now:

    awk: cmd. line:3: warning: escape sequence `\@' treated as plain `@'

Fix this by removing the backslash.

Reported-by: SZEDER Gábor <redacted>
Signed-off-by: Patrick Steinhardt <redacted>
Signed-off-by: Junio C Hamano <redacted>