Flow-ignore .git and .sl

Summary:
When working in a Git or Sapling clone of Metro OSS, Flow finds files inside checkout metadata. If these are in a bad state, it gives inactionable local errors, eg:

```
Error ┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈ .git/sl/origbackups/packages/metro-file-map/src/__tests__/index-test.js:1710:35

Cannot use <...>
```

This ignores them.

Changelog: Internal

Differential Revision: D109152977

fbshipit-source-id: ab2ad0e3f67e984ab545bf11289f0646fdd900b7

Author: robhogan

.flowconfig

@@ -1,6 +1,6 @@
 [ignore]
 <PROJECT_ROOT>/packages/.*/build/.*
-<PROJECT_ROOT>/\.hg/.*
+<PROJECT_ROOT>/\.\(hg\|sl\|git\)/.*
 # this transient dep bundles tests with their package, which flow attempts to parse
 # and crashes out as the test includes purposely malformed json
 <PROJECT_ROOT>\(/.*\)?/node_modules/resolve/test/.*