t4036-format-patch-signer-mime.sh: use the $( ... ) construct for command substitution

The Git CodingGuidelines prefer the $(...) construct for command
substitution instead of using the backquotes `...`.

The backquoted form is the traditional method for command
substitution, and is supported by POSIX.  However, all but the
simplest uses become complicated quickly.  In particular, embedded
command substitutions and/or the use of double quotes require
careful escaping with the backslash character.

The patch was generated by:

for _f in $(find . -name "*.sh")
do
   sed -i 's@`\(.*\)`@$(\1)@g' ${_f}
done

and then carefully proof-read.

Signed-off-by: Elia Pinto <gitter.spiros@gmail.com>
Reviewed-by: Matthieu Moy <Matthieu.Moy@imag.fr>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: devzero2000

t/t4036-format-patch-signer-mime.sh

@@ -42,7 +42,7 @@ test_expect_success 'attach and signoff do not duplicate mime headers' '
 
 	GIT_COMMITTER_NAME="はまの ふにおう" \
 	git format-patch -s --stdout -1 --attach >output &&
-	test `grep -ci ^MIME-Version: output` = 1
+	test $(grep -ci ^MIME-Version: output) = 1
 
 '