t/t5710-info-alternate.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/t5710-info-alternate.sh

@@ -21,7 +21,7 @@ test_valid_repo() {
 	test_line_count = 0 fsck.log
 }
 
-base_dir=`pwd`
+base_dir=$(pwd)
 
 test_expect_success 'preparing first repository' \
 'test_create_repo A && cd A &&