t/t3600-rm.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>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t3600-rm.sh

@@ -115,7 +115,7 @@ test_expect_success '"rm" command printed' '
 	git add test-file &&
 	git commit -m "add file for rm test" &&
 	git rm test-file > rm-output &&
-	test `grep "^rm " rm-output | wc -l` = 1 &&
+	test $(grep "^rm " rm-output | wc -l) = 1 &&
 	rm -f test-file rm-output &&
 	git commit -m "remove file from rm test"
 '
@@ -125,7 +125,7 @@ test_expect_success '"rm" command suppressed with --quiet' '
 	git add test-file &&
 	git commit -m "add file for rm --quiet test" &&
 	git rm --quiet test-file > rm-output &&
-	test `wc -l < rm-output` = 0 &&
+	test $(wc -l < rm-output) = 0 &&
 	rm -f test-file rm-output &&
 	git commit -m "remove file from rm --quiet test"
 '