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
   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/thunderbird-patch-inline/appp.sh

@@ -10,7 +10,7 @@ CONFFILE=~/.appprc
 
 SEP="-=-=-=-=-=-=-=-=-=# Don't remove this line #=-=-=-=-=-=-=-=-=-"
 if [ -e "$CONFFILE" ] ; then
-	LAST_DIR=`grep -m 1 "^LAST_DIR=" "${CONFFILE}"|sed -e 's/^LAST_DIR=//'`
+	LAST_DIR=$(grep -m 1 "^LAST_DIR=" "${CONFFILE}"|sed -e 's/^LAST_DIR=//')
 	cd "${LAST_DIR}"
 else
 	cd > /dev/null
@@ -25,11 +25,11 @@ fi
 
 cd - > /dev/null
 
-SUBJECT=`sed -n -e '/^Subject: /p' "${PATCH}"`
-HEADERS=`sed -e '/^'"${SEP}"'$/,$d' $1`
-BODY=`sed -e "1,/${SEP}/d" $1`
-CMT_MSG=`sed -e '1,/^$/d' -e '/^---$/,$d' "${PATCH}"`
-DIFF=`sed -e '1,/^---$/d' "${PATCH}"`
+SUBJECT=$(sed -n -e '/^Subject: /p' "${PATCH}")
+HEADERS=$(sed -e '/^'"${SEP}"'$/,$d' $1)
+BODY=$(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'`
@@ -48,7 +48,7 @@ if [ "x${BODY}x" != "xx" ] ; then
 fi
 echo "$DIFF" >> $1
 
-LAST_DIR=`dirname "${PATCH}"`
+LAST_DIR=$(dirname "${PATCH}")
 
 grep -v "^LAST_DIR=" "${CONFFILE}" > "${CONFFILE}_"
 echo "LAST_DIR=${LAST_DIR}" >> "${CONFFILE}_"