gitattributes: Do not export any .git paths in git archive

We have paths for `.gitattributes` itself, `.gitignore`, and
`.github`.  None should be exported in source archives.
diff --git a/.gitattributes b/.gitattributes
index 24fd9c2..3854b73 100644
--- a/.gitattributes
+++ b/.gitattributes
@@ -1,4 +1,4 @@
-.gitattributes   export-ignore
+.git*            export-ignore
 .hooks*          export-ignore
 
 # Custom attribute to mark sources as using our C code style.