git-revert.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: NElia Pinto <gitter.spiros@gmail.com>
Reviewed-by: NMatthieu Moy <Matthieu.Moy@imag.fr>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

contrib/examples/git-revert.sh

@@ -137,7 +137,7 @@ cherry-pick)
 		q
 	}'
 
-	logmsg=`git show -s --pretty=raw --encoding="$encoding" "$commit"`
+	logmsg=$(git show -s --pretty=raw --encoding="$encoding" "$commit")
 	set_author_env=`echo "$logmsg" |
 	LANG=C LC_ALL=C sed -ne "$pick_author_script"`
 	eval "$set_author_env"