test-sha1.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>
Reviewed-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

test-sha1.sh

@@ -6,13 +6,13 @@ dd if=/dev/zero bs=1048576 count=100 2>/dev/null |
 while read expect cnt pfx
 do
 	case "$expect" in '#'*) continue ;; esac
-	actual=`
+	actual=$(
 		{
 			test -z "$pfx" || echo "$pfx"
 			dd if=/dev/zero bs=1048576 count=$cnt 2>/dev/null |
 			perl -pe 'y/\000/g/'
 		} | ./test-sha1 $cnt
-	`
+	)
 	if test "$expect" = "$actual"
 	then
 		echo "OK: $expect $cnt $pfx"
@@ -51,14 +51,14 @@ exit
 
 while read cnt pfx
 do
-	actual=`
+	actual=$(
 		{
 			test -z "$pfx" || echo "$pfx"
 			dd if=/dev/zero bs=1048576 count=$cnt 2>/dev/null |
 			perl -pe 'y/\000/g/'
 		} | sha1sum |
 		sed -e 's/ .*//'
-	`
+	)
 	echo "$actual $cnt $pfx"
 done <<EOF
 0