git-merge.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>

contrib/examples/git-merge.sh

@@ -341,15 +341,15 @@ case "$use_strategies" in
 '')
 	case "$#" in
 	1)
-		var="`git config --get pull.twohead`"
+		var="$(git config --get pull.twohead)"
 		if test -n "$var"
 		then
 			use_strategies="$var"
 		else
 			use_strategies="$default_twohead_strategies"
 		fi ;;
 	*)
-		var="`git config --get pull.octopus`"
+		var="$(git config --get pull.octopus)"
 		if test -n "$var"
 		then
 			use_strategies="$var"