t/t6132-pathspec-exclude.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/t6132-pathspec-exclude.sh

@@ -7,7 +7,7 @@ test_description='test case exclude pathspec'
 test_expect_success 'setup' '
 	for p in file sub/file sub/sub/file sub/file2 sub/sub/sub/file sub2/file; do
 		if echo $p | grep /; then
-			mkdir -p `dirname $p`
+			mkdir -p $(dirname $p)
 		fi &&
 		: >$p &&
 		git add $p &&