Merge branch 'jc/countermand-format-attach'

The format.attach configuration variable lacked a way to override a
value defined in a lower-priority configuration file (e.g. the
system one) by redefining it in a higher-priority configuration
file.  Now, setting format.attach to an empty string means show the
patch inline in the e-mail message, without using MIME attachment.

This is a backward incompatible change.

* jc/countermand-format-attach:
  format.attach: allow empty value to disable multi-part messages

Documentation/config/format.txt

@@ -3,7 +3,8 @@ format.attach::
 	'format-patch'.  The value can also be a double quoted string
 	which will enable attachments as the default and set the
 	value as the boundary.  See the --attach option in
-	linkgit:git-format-patch[1].
+	linkgit:git-format-patch[1].  To countermand an earlier
+	value, set it to an empty string.
 
 format.from::
 	Provides the default value for the `--from` option to format-patch.

builtin/log.c

@@ -1007,6 +1007,8 @@ static int git_format_config(const char *var, const char *value, void *cb)
 	if (!strcmp(var, "format.attach")) {
 		if (value && *value)
 			default_attach = xstrdup(value);
+		else if (value && !*value)
+			FREE_AND_NULL(default_attach);
 		else
 			default_attach = xstrdup(git_version_string);
 		return 0;

t/t4014-format-patch.sh

@@ -2281,6 +2281,24 @@ test_expect_success 'format-patch --attach cover-letter only is non-multipart' '
 	test_line_count = 1 output
 '
 
+test_expect_success 'format-patch with format.attach' '
+	test_when_finished "rm -fr patches" &&
+	separator=attachment-separator &&
+	test_config format.attach "$separator" &&
+	filename=$(git format-patch -o patches -1) &&
+	grep "^Content-Type: multipart/.*$separator" "$filename"
+'
+
+test_expect_success 'format-patch with format.attach=disabled' '
+	test_when_finished "rm -fr patches" &&
+	separator=attachment-separator &&
+	test_config_global format.attach "$separator" &&
+	test_config format.attach "" &&
+	filename=$(git format-patch -o patches -1) &&
+	# The output should not even declare content type for text/plain.
+	! grep "^Content-Type: multipart/" "$filename"
+'
+
 test_expect_success '-c format.mboxrd format-patch' '
 	sp=" " &&
 	cat >msg <<-INPUT_END &&