t0030-stripspace.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/t0030-stripspace.sh

@@ -225,22 +225,22 @@ test_expect_success \
 
 test_expect_success \
     'text without newline at end should end with newline' '
-    test `printf "$ttt" | git stripspace | wc -l` -gt 0 &&
-    test `printf "$ttt$ttt" | git stripspace | wc -l` -gt 0 &&
-    test `printf "$ttt$ttt$ttt" | git stripspace | wc -l` -gt 0 &&
-    test `printf "$ttt$ttt$ttt$ttt" | git stripspace | wc -l` -gt 0
+    test $(printf "$ttt" | git stripspace | wc -l) -gt 0 &&
+    test $(printf "$ttt$ttt" | git stripspace | wc -l) -gt 0 &&
+    test $(printf "$ttt$ttt$ttt" | git stripspace | wc -l) -gt 0 &&
+    test $(printf "$ttt$ttt$ttt$ttt" | git stripspace | wc -l) -gt 0
 '
 
 # text plus spaces at the end:
 
 test_expect_success \
     'text plus spaces without newline at end should end with newline' '
-    test `printf "$ttt$sss" | git stripspace | wc -l` -gt 0 &&
-    test `printf "$ttt$ttt$sss" | git stripspace | wc -l` -gt 0 &&
-    test `printf "$ttt$ttt$ttt$sss" | git stripspace | wc -l` -gt 0 &&
-    test `printf "$ttt$sss$sss" | git stripspace | wc -l` -gt 0 &&
-    test `printf "$ttt$ttt$sss$sss" | git stripspace | wc -l` -gt 0 &&
-    test `printf "$ttt$sss$sss$sss" | git stripspace | wc -l` -gt 0
+    test $(printf "$ttt$sss" | git stripspace | wc -l) -gt 0 &&
+    test $(printf "$ttt$ttt$sss" | git stripspace | wc -l) -gt 0 &&
+    test $(printf "$ttt$ttt$ttt$sss" | git stripspace | wc -l) -gt 0 &&
+    test $(printf "$ttt$sss$sss" | git stripspace | wc -l) -gt 0 &&
+    test $(printf "$ttt$ttt$sss$sss" | git stripspace | wc -l) -gt 0 &&
+    test $(printf "$ttt$sss$sss$sss" | git stripspace | wc -l) -gt 0
 '
 
 test_expect_success \