t4038-diff-combined.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/t4038-diff-combined.sh

@@ -94,7 +94,7 @@ test_expect_success 'setup for --cc --raw' '
 	blob=$(echo file | git hash-object --stdin -w) &&
 	base_tree=$(echo "100644 blob $blob	file" | git mktree) &&
 	trees= &&
-	for i in `test_seq 1 40`
+	for i in $(test_seq 1 40)
 	do
 		blob=$(echo file$i | git hash-object --stdin -w) &&
 		trees="$trees$(echo "100644 blob $blob	file" | git mktree)$LF"