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

@@ -15,6 +15,7 @@ import { OkCancelButtonGroup } from '../dialog/ok-cancel-button-group'
 import untildify from 'untildify'
 import { showOpenDialog } from '../main-process-proxy'
 import { Ref } from '../lib/ref'
+import { isGitRepository } from './create-repository'
 
 interface IAddExistingRepositoryProps {
   readonly dispatcher: Dispatcher
@@ -218,9 +219,9 @@ 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 () => {