t/t7504-commit-msg-hook.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
	perl -i -pe 'BEGIN{undef $/;} s/`(.+?)`/\$(\1)/smg'  "${_f}"
done

and then carefully proof-read.

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

t/t7504-commit-msg-hook.sh

@@ -179,7 +179,7 @@ EOF
 chmod +x "$HOOK"
 
 commit_msg_is () {
-	test "`git log --pretty=format:%s%b -1`" = "$1"
+	test "$(git log --pretty=format:%s%b -1)" = "$1"
 }
 
 test_expect_success 'hook edits commit message' '