t/t3030-merge-recursive.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>
Reviewed-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t3030-merge-recursive.sh

@@ -263,7 +263,7 @@ test_expect_success 'setup 8' '
 	test_ln_s_add e a &&
 	test_tick &&
 	git commit -m "rename a->e, symlink a->e" &&
-	oln=`printf e | git hash-object --stdin`
+	oln=$(printf e | git hash-object --stdin)
 '
 
 test_expect_success 'setup 9' '