t0010-racy-git.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>

t/t0010-racy-git.sh

@@ -14,7 +14,7 @@ do
 	git update-index --add infocom
 	echo xyzzy >infocom
 
-	files=`git diff-files -p`
+	files=$(git diff-files -p)
 	test_expect_success \
 	"Racy GIT trial #$trial part A" \
 	'test "" != "$files"'
@@ -23,7 +23,7 @@ do
 	echo xyzzy >cornerstone
 	git update-index --add cornerstone
 
-	files=`git diff-files -p`
+	files=$(git diff-files -p)
 	test_expect_success \
 	"Racy GIT trial #$trial part B" \
 	'test "" != "$files"'