[PATCH] format.attach: allow empty value to disable multi-part messages

[Junio C Hamano <gitster@pobox.com>]

When a lower precedence configuration file (e.g. /etc/gitconfig)
defines format.attach in any way, there was no way to disable it in
a more specific configuration file (e.g. $HOME/.gitconfig).

Change the behaviour of setting it to an empty string.  It used to
mean that the result is still a multipart message with only dashes
used as a multi-part separator, but now it resets the setting to
the default (which would be to give an inline patch, unless other
command line options are in effect).

Signed-off-by: Junio C Hamano <gitster@pobox.com>
---

 * This is a backward incompatible change, so we may not take it in the
   end.  We'll see.

 Documentation/config/format.txt |  3 ++-
 builtin/log.c                   |  2 ++
 t/t4014-format-patch.sh         | 18 ++++++++++++++++++
 3 files changed, 22 insertions(+), 1 deletion(-)

diff --git a/Documentation/config/format.txt b/Documentation/config/format.txt
index 3bd78269e2..73678d88a1 100644
--- a/Documentation/config/format.txt
+++ b/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.
diff --git a/builtin/log.c b/builtin/log.c
index 04412dd9c9..a70fba198f 100644
--- a/builtin/log.c
+++ b/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;
diff --git a/t/t4014-format-patch.sh b/t/t4014-format-patch.sh
index 012f155e10..f3313b8c58 100755
--- a/t/t4014-format-patch.sh
+++ b/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 &&
-- 
2.39.2-501-gd9d677b2d8