t/t7103-reset-bare.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/t7103-reset-bare.sh

@@ -63,7 +63,7 @@ test_expect_success '"mixed" reset is not allowed in bare' '
 
 test_expect_success '"soft" reset is allowed in bare' '
 	git reset --soft HEAD^ &&
-	test "`git show --pretty=format:%s | head -n 1`" = "one"
+	test "$(git show --pretty=format:%s | head -n 1)" = "one"
 '
 
 test_done