t4013-diff-various.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/t4013-diff-various.sh

@@ -107,14 +107,14 @@ test_expect_success setup '
 +*++ [initial] Initial
 EOF
 
-V=`git version | sed -e 's/^git version //' -e 's/\./\\./g'`
+V=$(git version | sed -e 's/^git version //' -e 's/\./\\./g')
 while read cmd
 do
 	case "$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" $test_count)
 	expect="$TEST_DIRECTORY/t4013/diff.$test"
 	actual="$pfx-diff.$test"