t0300-credentials.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
   sed -i 's@`\(.*\)`@$(\1)@g' ${_f}
done

and then carefully proof-read.

Signed-off-by: Elia Pinto <gitter.spiros@gmail.com>
Reviewed-by: Matthieu Moy <Matthieu.Moy@imag.fr>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t0300-credentials.sh

@@ -6,7 +6,7 @@ test_description='basic credential helper tests'
 
 test_expect_success 'setup helper scripts' '
 	cat >dump <<-\EOF &&
-	whoami=`echo $0 | sed s/.*git-credential-//`
+	whoami=$(echo $0 | sed s/.*git-credential-//)
 	echo >&2 "$whoami: $*"
 	OIFS=$IFS
 	IFS==