t/t1700-split-index.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/t1700-split-index.sh

@@ -55,7 +55,7 @@ test_expect_success 'disable split index' '
 EOF
 	test_cmp ls-files.expect ls-files.actual &&
 
-	BASE=`test-dump-split-index .git/index | grep "^own" | sed "s/own/base/"` &&
+	BASE=$(test-dump-split-index .git/index | grep "^own" | sed "s/own/base/") &&
 	test-dump-split-index .git/index | sed "/^own/d" >actual &&
 	cat >expect <<EOF &&
 not a split index