t/t5515-fetch-merge-logic.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>
diff --git a/t/t5515-fetch-merge-logic.sh b/t/t5515-fetch-merge-logic.sh
index dbb927d..36b0dbc 100755
--- a/t/t5515-fetch-merge-logic.sh
+++ b/t/t5515-fetch-merge-logic.sh
@@ -128,8 +128,8 @@
 	case "$cmd" in
 	'' | '#'*) continue ;;
 	esac
-	test=`echo "$cmd" | sed -e 's|[/ ][/ ]*|_|g'`
-	pfx=`printf "%04d" $test_count`
+	test=$(echo "$cmd" | sed -e 's|[/ ][/ ]*|_|g')
+	pfx=$(printf "%04d" $test_count)
 	expect_f="$TEST_DIRECTORY/t5515/fetch.$test"
 	actual_f="$pfx-fetch.$test"
 	expect_r="$TEST_DIRECTORY/t5515/refs.$test"