contrib/thunderbird-patch-inline/appp.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: NElia Pinto <gitter.spiros@gmail.com>
Reviewed-by: NJonathan Nieder <jrnieder@gmail.com>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

contrib/thunderbird-patch-inline/appp.sh

@@ -31,8 +31,8 @@ BODY=$(sed -e "1,/${SEP}/d" $1)
 CMT_MSG=$(sed -e '1,/^$/d' -e '/^---$/,$d' "${PATCH}")
 DIFF=$(sed -e '1,/^---$/d' "${PATCH}")
 
-CCS=`echo -e "$CMT_MSG\n$HEADERS" | sed -n -e 's/^Cc: \(.*\)$/\1,/gp' \
-	-e 's/^Signed-off-by: \(.*\)/\1,/gp'`
+CCS=$(echo -e "$CMT_MSG\n$HEADERS" | sed -n -e 's/^Cc: \(.*\)$/\1,/gp' \
+	-e 's/^Signed-off-by: \(.*\)/\1,/gp')
 
 echo "$SUBJECT" > $1
 echo "Cc: $CCS" >> $1