t/t9001-send-email.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/t9001-send-email.sh

@@ -36,7 +36,7 @@ clean_fake_sendmail () {
 }
 
 test_expect_success $PREREQ 'Extract patches' '
-	patches=`git format-patch -s --cc="One <one@example.com>" --cc=two@example.com -n HEAD^1`
+	patches=$(git format-patch -s --cc="One <one@example.com>" --cc=two@example.com -n HEAD^1)
 '
 
 # Test no confirm early to ensure remaining tests will not hang
@@ -1151,7 +1151,7 @@ test_expect_success $PREREQ '--no-bcc overrides sendemail.bcc' '
 '
 
 test_expect_success $PREREQ 'patches To headers are used by default' '
-	patch=`git format-patch -1 --to="bodies@example.com"` &&
+	patch=$(git format-patch -1 --to="bodies@example.com") &&
 	test_when_finished "rm $patch" &&
 	git send-email \
 		--dry-run \
@@ -1162,7 +1162,7 @@ test_expect_success $PREREQ 'patches To headers are used by default' '
 '
 
 test_expect_success $PREREQ 'patches To headers are appended to' '
-	patch=`git format-patch -1 --to="bodies@example.com"` &&
+	patch=$(git format-patch -1 --to="bodies@example.com") &&
 	test_when_finished "rm $patch" &&
 	git send-email \
 		--dry-run \
@@ -1175,8 +1175,8 @@ test_expect_success $PREREQ 'patches To headers are appended to' '
 '
 
 test_expect_success $PREREQ 'To headers from files reset each patch' '
-	patch1=`git format-patch -1 --to="bodies@example.com"` &&
-	patch2=`git format-patch -1 --to="other@example.com" HEAD~` &&
+	patch1=$(git format-patch -1 --to="bodies@example.com") &&
+	patch2=$(git format-patch -1 --to="other@example.com" HEAD~) &&
 	test_when_finished "rm $patch1 && rm $patch2" &&
 	git send-email \
 		--dry-run \
@@ -1488,7 +1488,7 @@ test_cover_addresses () {
 	clean_fake_sendmail &&
 	rm -fr outdir &&
 	git format-patch --cover-letter -2 -o outdir &&
-	cover=`echo outdir/0000-*.patch` &&
+	cover=$(echo outdir/0000-*.patch) &&
 	mv $cover cover-to-edit.patch &&
 	perl -pe "s/^From:/$header: extra\@address.com\nFrom:/" cover-to-edit.patch >"$cover" &&
 	git send-email \