t/lib-httpd.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: NElia Pinto <gitter.spiros@gmail.com>
Reviewed-by: NJonathan Nieder <jrnieder@gmail.com>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

t/lib-httpd.sh

@@ -98,8 +98,8 @@ then
 	test_skip_or_die $GIT_TEST_HTTPD "no web server found at '$LIB_HTTPD_PATH'"
 fi
 
-HTTPD_VERSION=`$LIB_HTTPD_PATH -v | \
-	sed -n 's/^Server version: Apache\/\([0-9]*\)\..*$/\1/p; q'`
+HTTPD_VERSION=$($LIB_HTTPD_PATH -v | \
+	sed -n 's/^Server version: Apache\/\([0-9]*\)\..*$/\1/p; q')
 
 if test -n "$HTTPD_VERSION"
 then