contrib/examples/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
	perl -i -pe 'BEGIN{undef $/;} s/`(.+?)`/\$(\1)/smg'  "${_f}"
done

and then carefully proof-read.

Signed-off-by: Elia Pinto <gitter.spiros@gmail.com>
Reviewed-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

contrib/examples/git-revert.sh

@@ -138,8 +138,8 @@ cherry-pick)
 	}'
 
 	logmsg=$(git show -s --pretty=raw --encoding="$encoding" "$commit")
-	set_author_env=`echo "$logmsg" |
-	LANG=C LC_ALL=C sed -ne "$pick_author_script"`
+	set_author_env=$(echo "$logmsg" |
+	LANG=C LC_ALL=C sed -ne "$pick_author_script")
 	eval "$set_author_env"
 	export GIT_AUTHOR_NAME
 	export GIT_AUTHOR_EMAIL
@@ -160,9 +160,9 @@ cherry-pick)
 esac >.msg
 
 eval GITHEAD_$head=HEAD
-eval GITHEAD_$next='`git show -s \
+eval GITHEAD_$next='$(git show -s \
 	--pretty=oneline --encoding="$encoding" "$commit" |
-	sed -e "s/^[^ ]* //"`'
+	sed -e "s/^[^ ]* //")'
 export GITHEAD_$head GITHEAD_$next
 
 # This three way merge is an interesting one.  We are at