t/t5537-fetch-shallow.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/t5537-fetch-shallow.sh

@@ -98,7 +98,7 @@ EOF
 test_expect_success 'fetch something upstream has but hidden by clients shallow boundaries' '
 	# the blob "1" is available in .git but hidden by the
 	# shallow2/.git/shallow and it should be resent
-	! git --git-dir=shallow2/.git cat-file blob `echo 1|git hash-object --stdin` >/dev/null &&
+	! git --git-dir=shallow2/.git cat-file blob $(echo 1|git hash-object --stdin) >/dev/null &&
 	echo 1 >1.t &&
 	git add 1.t &&
 	git commit -m add-1-back &&
@@ -114,7 +114,7 @@ add-1-back
 EOF
 	test_cmp expect actual
 	) &&
-	git --git-dir=shallow2/.git cat-file blob `echo 1|git hash-object --stdin` >/dev/null
+	git --git-dir=shallow2/.git cat-file blob $(echo 1|git hash-object --stdin) >/dev/null
 
 '