t/t6001-rev-list-graft.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/t6001-rev-list-graft.sh

@@ -10,15 +10,15 @@ test_expect_success setup '
 	echo >subdir/fileB fileB &&
 	git add fileA subdir/fileB &&
 	git commit -a -m "Initial in one history." &&
-	A0=`git rev-parse --verify HEAD` &&
+	A0=$(git rev-parse --verify HEAD) &&
 
 	echo >fileA fileA modified &&
 	git commit -a -m "Second in one history." &&
-	A1=`git rev-parse --verify HEAD` &&
+	A1=$(git rev-parse --verify HEAD) &&
 
 	echo >subdir/fileB fileB modified &&
 	git commit -a -m "Third in one history." &&
-	A2=`git rev-parse --verify HEAD` &&
+	A2=$(git rev-parse --verify HEAD) &&
 
 	rm -f .git/refs/heads/master .git/index &&
 
@@ -26,15 +26,15 @@ test_expect_success setup '
 	echo >subdir/fileB fileB again &&
 	git add fileA subdir/fileB &&
 	git commit -a -m "Initial in alternate history." &&
-	B0=`git rev-parse --verify HEAD` &&
+	B0=$(git rev-parse --verify HEAD) &&
 
 	echo >fileA fileA modified in alternate history &&
 	git commit -a -m "Second in alternate history." &&
-	B1=`git rev-parse --verify HEAD` &&
+	B1=$(git rev-parse --verify HEAD) &&
 
 	echo >subdir/fileB fileB modified in alternate history &&
 	git commit -a -m "Third in alternate history." &&
-	B2=`git rev-parse --verify HEAD` &&
+	B2=$(git rev-parse --verify HEAD) &&
 	: done
 '