debian: rename directory debian to debian-stable

.gitattributes

@@ -1,7 +1,7 @@
 # files/directories excluded from tarballs
 
 .git*               export-ignore
-debian              export-ignore
+debian-stable       export-ignore
 weechat.spec        export-ignore
 .mailmap            export-ignore
 tests/ubuntu/       export-ignore

.gitignore

@@ -28,7 +28,7 @@ config.rpath
 config.status
 config.sub
 configure
-debian/*.log
+debian-stable/*.log
 depcomp
 insert-header.sin
 install-sh