t4057-diff-combined-paths.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/t4057-diff-combined-paths.sh

@@ -5,7 +5,7 @@ test_description='combined diff show only paths that are different to all parent
 . ./test-lib.sh
 
 # verify that diffc.expect matches output of
-# `git diff -c --name-only HEAD HEAD^ HEAD^2`
+# $(git diff -c --name-only HEAD HEAD^ HEAD^2)
 diffc_verify () {
 	git diff -c --name-only HEAD HEAD^ HEAD^2 >diffc.actual &&
 	test_cmp diffc.expect diffc.actual