Fix regression in incremental Watchman queries on Windows (#1231)

Summary:
Pull Request resolved: #1231

D52513808 / #1210 introduced a startup regression in Watchman crawling on Windows where a clock key was incorrectly calculated due to more strict behaviour around path separators.

This meant the clock was not matched with cached state, and the crawler would fall back to a "glob" query, rather than the (typically much cheaper) incremental "since" query.

The bug isn't in the new `RootPathUtils` itself, which already states its assumptions (including `"All input path separators must be system-native."`), but rather in the failure to properly normalise a particular response from Watchman (which returns posix separators, even on Windows) to Metro's system-native paths, before using `pathUtils.absoluteToNormal`. Note that we already normalise other responses from Watchman to system separators.

Before D52513808 / #1210 we would've deemed the path too complex to handle and fallen back to `path.relative`, which is much slower but more forgiving.

Changelog
```
* **[Fix]**: Fix startup time regression when using Watchman on Windows with a warm cache.
```

Reviewed By: motiz88

Differential Revision: D54592646

fbshipit-source-id: de91b1a724fcec470e93650ec9c6adce8c148aac

packages/metro-file-map/src/crawlers/watchman/index.js

@@ -31,7 +31,7 @@ import {performance} from 'perf_hooks';
 const watchman = require('fb-watchman');
 
 type WatchmanRoots = Map<
-  string,
+  string, // Posix-separated absolute path
   $ReadOnly<{directoryFilters: Array<string>, watcher: string}>,
 >;
 
@@ -181,7 +181,7 @@ module.exports = async function watchmanCrawl({
 
     await Promise.all(
       Array.from(rootProjectDirMappings).map(
-        async ([root, {directoryFilters, watcher}], index) => {
+        async ([posixSeparatedRoot, {directoryFilters, watcher}], index) => {
           // Jest is only going to store one type of clock; a string that
           // represents a local clock. However, the Watchman crawler supports
           // a second type of clock that can be written by automation outside of
@@ -191,7 +191,11 @@ module.exports = async function watchmanCrawl({
           // By using scm queries, we can create the haste map on a different
           // system and import it, transforming the clock into a local clock.
           const since = previousState.clocks.get(
-            normalizePathSeparatorsToPosix(pathUtils.absoluteToNormal(root)),
+            normalizePathSeparatorsToPosix(
+              pathUtils.absoluteToNormal(
+                normalizePathSeparatorsToSystem(posixSeparatedRoot),
+              ),
+            ),
           );
 
           perfLogger?.annotate({
@@ -218,7 +222,7 @@ module.exports = async function watchmanCrawl({
           perfLogger?.point(`watchmanCrawl/query_${index}_start`);
           const response = await cmd<WatchmanQueryResponse>(
             'query',
-            root,
+            posixSeparatedRoot,
             query,
           );
           perfLogger?.point(`watchmanCrawl/query_${index}_end`);
@@ -232,7 +236,7 @@ module.exports = async function watchmanCrawl({
             isFresh = isFresh || response.is_fresh_instance;
           }
 
-          results.set(root, response);
+          results.set(posixSeparatedRoot, response);
         },
       ),
     );