git-pull.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>

git-pull.sh

@@ -108,7 +108,7 @@ do
 	-s|--s|--st|--str|--stra|--strat|--strate|--strateg|--strategy)
 		case "$#,$1" in
 		*,*=*)
-			strategy=`expr "z$1" : 'z-[^=]*=\(.*\)'` ;;
+			strategy=$(expr "z$1" : 'z-[^=]*=\(.*\)') ;;
 		1,*)
 			usage ;;
 		*)