t/t5303-pack-corruption-resilience.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>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t5303-pack-corruption-resilience.sh

@@ -32,23 +32,23 @@ create_test_files() {
 create_new_pack() {
     rm -rf .git &&
     git init &&
-    blob_1=`git hash-object -t blob -w file_1` &&
-    blob_2=`git hash-object -t blob -w file_2` &&
-    blob_3=`git hash-object -t blob -w file_3` &&
-    pack=`printf "$blob_1\n$blob_2\n$blob_3\n" |
-          git pack-objects $@ .git/objects/pack/pack` &&
+    blob_1=$(git hash-object -t blob -w file_1) &&
+    blob_2=$(git hash-object -t blob -w file_2) &&
+    blob_3=$(git hash-object -t blob -w file_3) &&
+    pack=$(printf "$blob_1\n$blob_2\n$blob_3\n" |
+          git pack-objects $@ .git/objects/pack/pack) &&
     pack=".git/objects/pack/pack-${pack}" &&
     git verify-pack -v ${pack}.pack
 }
 
 do_repack() {
-    pack=`printf "$blob_1\n$blob_2\n$blob_3\n" |
-          git pack-objects $@ .git/objects/pack/pack` &&
+    pack=$(printf "$blob_1\n$blob_2\n$blob_3\n" |
+          git pack-objects $@ .git/objects/pack/pack) &&
     pack=".git/objects/pack/pack-${pack}"
 }
 
 do_corrupt_object() {
-    ofs=`git show-index < ${pack}.idx | grep $1 | cut -f1 -d" "` &&
+    ofs=$(git show-index < ${pack}.idx | grep $1 | cut -f1 -d" ") &&
     ofs=$(($ofs + $2)) &&
     chmod +w ${pack}.pack &&
     dd of=${pack}.pack bs=1 conv=notrunc seek=$ofs &&