Cache micromatch in SearchSource globsToMatcher

I was profiling some Jest runs at Airbnb and noticed that on my
MacBook Pro, we can spend over 2 seconds at Jest startup time in
SearchSource getTestPaths. I believe that this will grow as the size
of the codebase increases.

Looking at the call stacks, it appears to be calling micromatch
repeatedly, which calls picomatch, which builds a regex out of the
globs. It seems that the parsing and regex building also triggers the
garbage collector frequently.

Upon testing, I noticed that the globs don't actually change between
these calls, so we can safe a bunch of work by making a micromatch
matcher and reusing that function if the globs haven't changed. In my
basic testing, it is always called with the same set of globs, so I
went with a very simple last-used memoization instead of a more robust
caching solution.

In my profiling of this change locally, this brings down the time of
startRun from about 2000ms to about 200ms.

CHANGELOG.md

@@ -22,6 +22,8 @@
 
 ### Performance
 
+- `[jest-core]` Cache micromatch in SearchSource globsToMatcher
+
 ## 26.0.1
 
 ### Fixes

packages/jest-core/src/SearchSource.ts

@@ -37,8 +37,16 @@ export type TestSelectionConfig = {
   watch?: boolean;
 };
 
-const globsToMatcher = (globs: Array<Config.Glob>) => (path: Config.Path) =>
-  micromatch([replacePathSepForGlob(path)], globs, {dot: true}).length > 0;
+let prevGlobs: Array<Config.Glob>;
+let prevGlobsMatchers: ((str: string) => boolean)[];
+const globsToMatcher = (globs: Array<Config.Glob>) => {
+  if (prevGlobs !== globs) {
+    prevGlobs = globs;
+    prevGlobsMatchers = globs.map(glob => micromatch.matcher(glob, {dot: true}));
+  }
+
+  return (path: Config.Path) => prevGlobsMatchers.some(isMatch => isMatch(replacePathSepForGlob(path)));
+}
 
 const regexToMatcher = (testRegex: Config.ProjectConfig['testRegex']) => (
   path: Config.Path,