check-builtins.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>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

check-builtins.sh

@@ -14,8 +14,8 @@ sort |
     bad=0
     while read builtin
     do
-	base=`expr "$builtin" : 'git-\(.*\)'`
-	x=`sed -ne 's/.*{ "'$base'", \(cmd_[^, ]*\).*/'$base'	\1/p' git.c`
+	base=$(expr "$builtin" : 'git-\(.*\)')
+	x=$(sed -ne 's/.*{ "'$base'", \(cmd_[^, ]*\).*/'$base'	\1/p' git.c)
 	if test -z "$x"
 	then
 		echo "$base is builtin but not listed in git.c command list"