git-send-email.perl: Handle shell metacharacters in $EDITOR properly

This fixes the git-send-perl semantics for launching an editor when
$GIT_EDITOR (or friends) contains shell metacharacters to match
launch_editor() in builtin-tag.c. If we use the current approach
(sh -c '$0 $@' "$EDITOR" files ...), we see it fails when $EDITOR has
shell metacharacters:

  $ sh -x -c '$0 $@' "$VISUAL" "foo"
  + "$FAKE_EDITOR" foo
  "$FAKE_EDITOR": 1: "$FAKE_EDITOR": not found

Whereas builtin-tag.c will invoke sh -c "$EDITOR \"$@\"".

Thus, this patch changes git-send-email.perl to use the same method as the
C utilities, and additionally updates t/t9001-send-email.sh to test for
this bug.

Signed-off-by: Bryan Donlan <bdonlan@fushizen.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

git-send-email.perl

@@ -512,7 +512,7 @@ EOT
 	close(C);
 
 	my $editor = $ENV{GIT_EDITOR} || Git::config(@repo, "core.editor") || $ENV{VISUAL} || $ENV{EDITOR} || "vi";
-	system('sh', '-c', '$0 $@', $editor, $compose_filename);
+	system('sh', '-c', $editor.' "$@"', $editor, $compose_filename);
 
 	open(C2,">",$compose_filename . ".final")
 		or die "Failed to open $compose_filename.final : " . $!;

t/t9001-send-email.sh

@@ -139,15 +139,19 @@ test_expect_success 'Valid In-Reply-To when prompting' '
 
 test_expect_success 'setup fake editor' '
 	(echo "#!/bin/sh" &&
-	 echo "echo fake edit >>\$1"
+	 echo "echo fake edit >>\"\$1\""
 	) >fake-editor &&
 	chmod +x fake-editor
 '
 
+FAKE_EDITOR="$(pwd)/fake-editor"
+export FAKE_EDITOR
+GIT_EDITOR='"$FAKE_EDITOR"'
+export GIT_EDITOR
+
 test_expect_success '--compose works' '
 	clean_fake_sendmail &&
 	echo y | \
-		GIT_EDITOR=$(pwd)/fake-editor \
 		GIT_SEND_EMAIL_NOTTY=1 \
 		git send-email \
 		--compose --subject foo \