t/t7406-submodule-update.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/t7406-submodule-update.sh

@@ -14,8 +14,8 @@ submodule and "git submodule update --rebase/--merge" does not detach the HEAD.
 
 compare_head()
 {
-    sha_master=`git rev-list --max-count=1 master`
-    sha_head=`git rev-list --max-count=1 HEAD`
+    sha_master=$(git rev-list --max-count=1 master)
+    sha_head=$(git rev-list --max-count=1 HEAD)
 
     test "$sha_master" = "$sha_head"
 }