t3905-stash-include-untracked.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/t3905-stash-include-untracked.sh

@@ -96,8 +96,8 @@ test_expect_success 'stash pop after save --include-untracked leaves files untra
 	git stash pop &&
 	git status --porcelain >actual &&
 	test_cmp expect actual &&
-	test "1" = "`cat file2`" &&
-	test untracked = "`cat untracked/untracked`"
+	test "1" = "$(cat file2)" &&
+	test untracked = "$(cat untracked/untracked)"
 '
 
 git clean --force --quiet -d