t9138-git-svn-authors-prog.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/t9138-git-svn-authors-prog.sh

@@ -37,7 +37,7 @@ test_expect_success 'import authors with prog and file' '
 test_expect_success 'imported 6 revisions successfully' '
 	(
 		cd x
-		test "`git rev-list refs/remotes/git-svn | wc -l`" -eq 6
+		test "$(git rev-list refs/remotes/git-svn | wc -l)" -eq 6
 	)
 '