t7610-mergetool: use test_cmp instead of test $(cat file) = $txt

t7610-mergetool: use test_cmp instead of test $(cat file) = $txt

Fix that anti-pattern by a sequence of echo and test_cmp.

The patch was generated with this command:

   sed -i -e '/test.*(cat/s/^\(\t*\)test "..cat \(.*\))" = \(".*"\)\(.*\)/\1echo \3 >expect \&\&\n\1test_cmp expect \2\4/' t7610-mergetool.sh

This helps on Windows, where test_cmp avoids spawning a process when
there is no difference.

Signed-off-by: Johannes Sixt <redacted>
Signed-off-by: Junio C Hamano <redacted>