t/t7602-merge-octopus-many.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/t7602-merge-octopus-many.sh

@@ -19,7 +19,7 @@ test_expect_success 'setup' '
 		git add c$i.c &&
 		git commit -m c$i &&
 		git tag c$i &&
-		i=`expr $i + 1` || return 1
+		i=$(expr $i + 1) || return 1
 	done
 '
 
@@ -30,7 +30,7 @@ test_expect_success 'merge c1 with c2, c3, c4, ... c29' '
 	while test $i -le 30
 	do
 		refs="$refs c$i"
-		i=`expr $i + 1`
+		i=$(expr $i + 1)
 	done &&
 	git merge $refs &&
 	test "$(git rev-parse c1)" != "$(git rev-parse HEAD)" &&
@@ -38,14 +38,14 @@ test_expect_success 'merge c1 with c2, c3, c4, ... c29' '
 	while test $i -le 30
 	do
 		test "$(git rev-parse c$i)" = "$(git rev-parse HEAD^$i)" &&
-		i=`expr $i + 1` || return 1
+		i=$(expr $i + 1) || return 1
 	done &&
 	git diff --exit-code &&
 	i=1 &&
 	while test $i -le 30
 	do
 		test -f c$i.c &&
-		i=`expr $i + 1` || return 1
+		i=$(expr $i + 1) || return 1
 	done
 '