config: teach "git config --file -" to read from the standard input

The patch extends git config --file interface to allow read config from
stdin.

Editing stdin or setting value in stdin is an error.

Include by absolute path is allowed in stdin config, but not by relative
path.

Signed-off-by: Kirill A. Shutemov <kirill@shutemov.name>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

cache.h

@@ -1147,6 +1147,7 @@ extern int update_server_info(int);
 #define CONFIG_GENERIC_ERROR 7
 
 struct git_config_source {
+	unsigned int use_stdin:1;
 	const char *file;
 	const char *blob;
 };