git-rebase--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>

git-rebase--merge.sh

@@ -24,7 +24,7 @@ continue_merge () {
 		die "$resolvemsg"
 	fi
 
-	cmt=`cat "$state_dir/current"`
+	cmt=$(cat "$state_dir/current")
 	if ! git diff-index --quiet --ignore-submodules HEAD --
 	then
 		if ! git commit ${gpg_sign_opt:+"$gpg_sign_opt"} --no-verify -C "$cmt"
@@ -132,7 +132,7 @@ echo "$onto_name" > "$state_dir/onto_name"
 write_basic_state
 
 msgnum=0
-for cmt in `git rev-list --reverse --no-merges "$revisions"`
+for cmt in $(git rev-list --reverse --no-merges "$revisions")
 do
 	msgnum=$(($msgnum + 1))
 	echo "$cmt" > "$state_dir/cmt.$msgnum"