Make PR clones shallow and don't clone the branch first

Interim fix for travis-ci/travis-ci#7589

lib/travis/build/git/clone.rb

@@ -13,7 +13,6 @@ def apply
             clone_or_fetch
             delete_netrc
             sh.cd dir
-            fetch_ref if fetch_ref?
             checkout
           end
         end
@@ -22,36 +21,33 @@ def apply
 
           def clone_or_fetch
             sh.if "! -d #{dir}/.git" do
-              sh.cmd "git clone #{clone_args} #{data.source_url} #{dir}", assert: true, retry: true
+              sh.cmd "mkdir #{dir}", assert: true
+              sh.cd dir
+              # Need to manually perform the fetch steps - https://github.com/travis-ci/travis-ci/issues/7589
+              sh.cmd "git init", assert: true, timing: false
+              sh.cmd "git remote add origin #{data.source_url}", assert: true, timing: false
+              sh.cmd "git fetch #{fetch_args} origin +#{data.ref || branch}:", assert: false, retry: true
               if github?
                 sh.if "$? -ne 0" do
                   sh.echo "Failed to clone from GitHub.", ansi: :red
                   sh.echo "Checking GitHub status (https://status.github.com/api/last-message.json):"
                   sh.raw "curl -sL https://status.github.com/api/last-message.json | jq -r .[]"
                 end
               end
+              sh.cd ".."
             end
             sh.else do
               sh.cmd "git -C #{dir} fetch origin", assert: true, retry: true
               sh.cmd "git -C #{dir} reset --hard", assert: true, timing: false
             end
           end
 
-          def fetch_ref
-            sh.cmd "git fetch origin +#{data.ref}:", assert: true, retry: true
-          end
-
-          def fetch_ref?
-            !!data.ref
-          end
-
           def checkout
             sh.cmd "git checkout -qf #{data.pull_request ? 'FETCH_HEAD' : data.commit}", timing: false
           end
 
-          def clone_args
+          def fetch_args
             args = "--depth=#{depth}"
-            args << " --branch=#{branch}" unless data.ref
             args << " --quiet" if quiet?
             args
           end

spec/build/git/clone_spec.rb

@@ -82,21 +82,24 @@
   end
 
 
-  describe 'when the repository is not yet cloned' do
-    let(:args) { "--depth=#{depth} --branch=#{branch.shellescape}" }
-    let(:cmd)  { "git clone #{args} #{url} #{dir}" }
+  describe 'when the repository is not yet cloned and is a branch' do
+    let(:args) { "--depth=#{depth}" }
+    let(:cmd)  { "git fetch #{args} origin +#{branch.shellescape}:" }
     subject    { sexp_find(sexp, [:if, "! -d #{dir}/.git"]) }
 
-    let(:clone) { [:cmd, cmd, assert: true, echo: true, retry: true, timing: true] }
+    let(:init) { [:cmd, "git init", assert: true, echo: true] }
+    let(:fetch) { [:cmd, cmd, echo: true, retry: true, timing: true] }
 
     describe 'with no depth specified' do
-      it { should include_sexp clone }
+      it { should include_sexp init }
+      it { should include_sexp fetch }
     end
 
     describe 'with a custom depth' do
       let(:depth) { 1 }
       before { payload[:config][:git]['depth'] = depth }
-      it { should include_sexp clone }
+      it { should include_sexp init }
+      it { should include_sexp fetch }
     end
 
     describe 'with lfs_skip_smudge true' do
@@ -106,15 +109,15 @@
 
     describe 'escapes the branch name' do
       before { payload[:job][:branch] = 'foo->bar' }
-      it { should include_sexp clone }
+      it { should include_sexp fetch }
     end
 
     context 'when git.quiet is true' do
       before :each do
         payload[:config][:git].merge!({ quiet: true })
       end
-      let(:args) { "--depth=#{depth} --branch=#{branch.shellescape} --quiet" }
-      it { should include_sexp clone }
+      let(:args) { "--depth=#{depth} --quiet" }
+      it { should include_sexp fetch }
     end
   end
 
@@ -129,7 +132,8 @@
   end
 
   let(:cd)            { [:cd,  'travis-ci/travis-ci', echo: true] }
-  let(:fetch_ref)     { [:cmd, %r(git fetch origin \+[\w/]+:), assert: true, echo: true, retry: true, timing: true] }
+  let(:fetch_args)    { "--depth=#{depth}" }
+  let(:fetch_ref)     { [:cmd, %r(git fetch #{fetch_args} origin \+[\w/]+:), echo: true, retry: true, timing: true] }
   let(:checkout_push) { [:cmd, 'git checkout -qf 313f61b', assert: true, echo: true] }
   let(:checkout_pull) { [:cmd, 'git checkout -qf FETCH_HEAD', assert: true, echo: true] }
 
@@ -140,10 +144,6 @@
     it { should include_sexp fetch_ref }
   end
 
-  describe 'with no ref given' do
-    it { should_not include_sexp fetch_ref }
-  end
-
   describe 'checks out the given commit for a push request' do
     before { payload[:job][:pull_request] = false }
     it { should include_sexp checkout_push }