t3910-mac-os-precompose.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>

t/t3910-mac-os-precompose.sh

@@ -14,13 +14,13 @@ then
 fi
 
 # create utf-8 variables
-Adiarnfc=`printf '\303\204'`
-Adiarnfd=`printf 'A\314\210'`
+Adiarnfc=$(printf '\303\204')
+Adiarnfd=$(printf 'A\314\210')
 
-Odiarnfc=`printf '\303\226'`
-Odiarnfd=`printf 'O\314\210'`
-AEligatu=`printf '\303\206'`
-Invalidu=`printf '\303\377'`
+Odiarnfc=$(printf '\303\226')
+Odiarnfd=$(printf 'O\314\210')
+AEligatu=$(printf '\303\206')
+Invalidu=$(printf '\303\377')
 
 
 #Create a string with 255 bytes (decomposed)
@@ -35,7 +35,7 @@ Alongc=$Alongc$Alongc$Alongc$Alongc$Alongc           #250 Byte
 Alongc=$Alongc$AEligatu$AEligatu                     #254 Byte
 
 test_expect_success "detect if nfd needed" '
-	precomposeunicode=`git config core.precomposeunicode` &&
+	precomposeunicode=$(git config core.precomposeunicode) &&
 	test "$precomposeunicode" = true &&
 	git config core.precomposeunicode true
 '
@@ -146,7 +146,7 @@ test_expect_success "respect git config --global core.precomposeunicode" '
 	git config --global core.precomposeunicode true &&
 	rm -rf .git &&
 	git init &&
-	precomposeunicode=`git config core.precomposeunicode` &&
+	precomposeunicode=$(git config core.precomposeunicode) &&
 	test "$precomposeunicode" = "true"
 '