t/t1511-rev-parse-caret.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>

t/t1511-rev-parse-caret.sh

@@ -6,11 +6,11 @@ test_description='tests for ref^{stuff}'
 
 test_expect_success 'setup' '
 	echo blob >a-blob &&
-	git tag -a -m blob blob-tag `git hash-object -w a-blob` &&
+	git tag -a -m blob blob-tag $(git hash-object -w a-blob) &&
 	mkdir a-tree &&
 	echo moreblobs >a-tree/another-blob &&
 	git add . &&
-	TREE_SHA1=`git write-tree` &&
+	TREE_SHA1=$(git write-tree) &&
 	git tag -a -m tree tree-tag "$TREE_SHA1" &&
 	git commit -m Initial &&
 	git tag -a -m commit commit-tag &&