lib-credential.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/lib-credential.sh

@@ -281,7 +281,7 @@ helper_test_timeout() {
 cat >askpass <<\EOF
 #!/bin/sh
 echo >&2 askpass: $*
-what=`echo $1 | cut -d" " -f1 | tr A-Z a-z | tr -cd a-z`
+what=$(echo $1 | cut -d" " -f1 | tr A-Z a-z | tr -cd a-z)
 echo "askpass-$what"
 EOF
 chmod +x askpass