editorconfig: Add .git* section

Configuration files of `git` are managed by the tool and thus are
tab-indented. If a developer happens to edit such a file manually, the
indentation would become inconsistent.

Add an explicit section for such files and set the indentation to tabs.

.editorconfig

@@ -10,3 +10,6 @@ insert_final_newline = true
 
 [*.md]
 trim_trailing_whitespace = false
+
+[.git*]
+indent_style = tab