allow tilde character when specifying a local repository (#316)

There already exists a function that will convert a tilde path to an absolute path. It was originally used for this purpose, but the functionality was removed during a commit that changed which function was used to validate git repositories.

This reinstates that functionality and allows us to type a ~/ tilde path to get our home directories when typing in a path.

app/src/ui/add-repository/add-existing-repository.tsx

@@ -1,7 +1,11 @@
 import * as React from 'react'
 import * as Path from 'path'
 import { Dispatcher } from '../dispatcher'
-import { addGlobalConfigValueIfMissing, getRepositoryType } from '../../lib/git'
+import {
+  addGlobalConfigValueIfMissing,
+  getRepositoryType,
+  isGitRepository,
+} from '../../lib/git'
 import { Button } from '../lib/button'
 import { TextBox } from '../lib/text-box'
 import { Row } from '../lib/row'
@@ -218,9 +222,9 @@ export class AddExistingRepository extends React.Component<
   }
 
   private onPathChanged = async (path: string) => {
-    if (this.state.path !== path) {
-      this.updatePath(path)
-    }
+    const isRepository = await isGitRepository(this.resolvedPath(path))
+
+    this.setState({ path, isRepository })
   }
 
   private showFilePicker = async () => {