Resolve Git dependencies offline if tarball is available. (#3000)

When running yarn offline, Git dependencies get trapped in
hasHTTPCapabiity even though resolveOverHTTP defaults to returning
the tarball. This change allows us to bypass that step altogether
if the tarball already exists.

Author: Raymond Wang

src/resolvers/exotics/hosted-git-resolver.js

@@ -15,7 +15,7 @@ export type ExplodedFragment = {
   hash: string,
 };
 
-export function explodeHostedGitFragment(fragment: string, reporter: Reporter): ExplodedFragment {  
+export function explodeHostedGitFragment(fragment: string, reporter: Reporter): ExplodedFragment {
 
   const preParts = fragment.split('@');
   if (preParts.length > 2) {
@@ -39,7 +39,7 @@ export function explodeHostedGitFragment(fragment: string, reporter: Reporter):
     throw new MessageError(reporter.lang('invalidHostedGitFragment', fragment));
   }
 
-  const userParts = fragment.split('/');  
+  const userParts = fragment.split('/');
 
   if (userParts.length >= 2) {
 
@@ -140,11 +140,6 @@ export default class HostedGitResolver extends ExoticResolver {
   }
 
   async resolveOverHTTP(url: string): Promise<Manifest> {
-    const shrunk = this.request.getLocked('tarball');
-    if (shrunk) {
-      return shrunk;
-    }
-
     const commit = await this.getRefOverHTTP(url);
     const {config} = this;
 
@@ -201,6 +196,12 @@ export default class HostedGitResolver extends ExoticResolver {
   }
 
   async resolve(): Promise<Manifest> {
+    // If we already have the tarball, just return it without having to make any HTTP requests.
+    const shrunk = this.request.getLocked('tarball');
+    if (shrunk) {
+      return shrunk;
+    }
+
     const httpUrl = this.constructor.getGitHTTPUrl(this.exploded);
     const httpBaseUrl = this.constructor.getGitHTTPBaseUrl(this.exploded);
     const sshUrl = this.constructor.getGitSSHUrl(this.exploded);