git add -p: use non-zero exit code when the diff generation failed

The first thing `git add -p` does is to generate a diff. If this diff
cannot be generated, `git add -p` should not continue as if nothing
happened, but instead fail.

What we *actually* do here is much broader: we now verify for *every*
`run_cmd_pipe()` call that the spawned process actually succeeded.

Note that we have to change two callers in this patch, as we need to
store the spawned process' output in a local variable, which means that
the callers can no longer decide whether to interpret the `return <$fh>`
in array or in scalar context.

This bug was noticed while writing a test case for the diff.algorithm
feature, and we let that test case double as a regression test for this
fixed bug, too.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

git-add--interactive.perl

@@ -177,7 +177,9 @@ sub run_cmd_pipe {
 	} else {
 		my $fh = undef;
 		open($fh, '-|', @_) or die;
-		return <$fh>;
+		my @out = <$fh>;
+		close $fh || die "Cannot close @_ ($!)";
+		return @out;
 	}
 }
 
@@ -224,7 +226,7 @@ my $status_head = sprintf($status_fmt, __('staged'), __('unstaged'), __('path'))
 	sub get_empty_tree {
 		return $empty_tree if defined $empty_tree;
 
-		$empty_tree = run_cmd_pipe(qw(git hash-object -t tree /dev/null));
+		($empty_tree) = run_cmd_pipe(qw(git hash-object -t tree /dev/null));
 		chomp $empty_tree;
 		return $empty_tree;
 	}
@@ -1127,7 +1129,7 @@ aborted and the hunk is left unchanged.
 EOF2
 	close $fh;
 
-	chomp(my $editor = run_cmd_pipe(qw(git var GIT_EDITOR)));
+	chomp(my ($editor) = run_cmd_pipe(qw(git var GIT_EDITOR)));
 	system('sh', '-c', $editor.' "$@"', $editor, $hunkfile);
 
 	if ($? != 0) {

t/t3701-add-interactive.sh

@@ -530,7 +530,7 @@ test_expect_success 'diff.algorithm is passed to `git diff-files`' '
 	>file &&
 	git add file &&
 	echo changed >file &&
-	git -c diff.algorithm=bogus add -p 2>err &&
+	test_must_fail git -c diff.algorithm=bogus add -p 2>err &&
 	test_i18ngrep "error: option diff-algorithm accepts " err
 '