Add CodeBuild support

Add CodeBuild support based on the CODEBUILD_WEBHOOK_HEAD_REF environment variables as documented here:
https://docs.aws.amazon.com/codebuild/latest/userguide/build-env-ref-env-vars.html
Currently this is the only way to determine the branch name in CodeBuild.

Author: derwasp

src/GitVersionCore.Tests/BuildServers/CodeBuildTests.cs

@@ -0,0 +1,90 @@
+using System;
+using System.Collections.Generic;
+using System.IO;
+using System.Reflection;
+using GitVersion;
+using NUnit.Framework;
+using Shouldly;
+using GitVersion.BuildServers;
+using GitVersion.Common;
+using GitVersion.OutputVariables;
+
+namespace GitVersionCore.Tests.BuildServers
+{
+    [TestFixture]
+    public sealed class CodeBuildTests : TestBase
+    {
+        private IEnvironment environment;
+
+        [SetUp]
+        public void SetUp()
+        {
+            environment = new TestEnvironment();
+        }
+
+        [Test]
+        public void CorrectlyIdentifiesCodeBuildPresence()
+        {
+            environment.SetEnvironmentVariable(CodeBuild.HeadRefEnvironmentName, "a value");
+            var cb = new CodeBuild(environment);
+            cb.CanApplyToCurrentContext().ShouldBe(true);
+        }
+
+        [Test]
+        public void PicksUpBranchNameFromEnvironment()
+        {
+            environment.SetEnvironmentVariable(CodeBuild.HeadRefEnvironmentName, "refs/heads/master");
+            var cb = new CodeBuild(environment);
+            cb.GetCurrentBranch(false).ShouldBe("refs/heads/master");
+        }
+
+        [Test]
+        public void WriteAllVariablesToTheTextWriter()
+        {
+            var assemblyLocation = Path.GetDirectoryName(Assembly.GetExecutingAssembly().Location);
+            var f = Path.Combine(assemblyLocation, "codebuild_this_file_should_be_deleted.properties");
+
+            try
+            {
+                AssertVariablesAreWrittenToFile(f);
+            }
+            finally
+            {
+                File.Delete(f);
+            }
+        }
+
+        private void AssertVariablesAreWrittenToFile(string f)
+        {
+            var writes = new List<string>();
+            var semanticVersion = new SemanticVersion
+            {
+                Major = 1,
+                Minor = 2,
+                Patch = 3,
+                PreReleaseTag = "beta1",
+                BuildMetaData = "5"
+            };
+
+            semanticVersion.BuildMetaData.CommitDate = DateTimeOffset.Parse("2014-03-06 23:59:59Z");
+            semanticVersion.BuildMetaData.Sha = "commitSha";
+
+            var config = new TestEffectiveConfiguration();
+
+            var variables = VariableProvider.GetVariablesFor(semanticVersion, config, false);
+
+            var j = new CodeBuild(environment, f);
+
+            j.WriteIntegration(writes.Add, variables);
+
+            writes[1].ShouldBe("1.2.3-beta.1+5");
+
+            File.Exists(f).ShouldBe(true);
+
+            var props = File.ReadAllText(f);
+
+            props.ShouldContain("GitVersion_Major=1");
+            props.ShouldContain("GitVersion_Minor=2");
+        }
+    }
+}

src/GitVersionCore/BuildServers/BuildServerList.cs

@@ -45,7 +45,8 @@ public static void Init(IEnvironment environment)
                 new VsoAgent(environment),
                 new TravisCI(environment),
                 new EnvRun(environment),
-                new Drone(environment)
+                new Drone(environment),
+                new CodeBuild(environment)
             };
         }
     }

src/GitVersionCore/BuildServers/CodeBuild.cs

@@ -0,0 +1,48 @@
+using System;
+using System.IO;
+using GitVersion.Common;
+using GitVersion.OutputFormatters;
+using GitVersion.OutputVariables;
+
+namespace GitVersion.BuildServers
+{
+    public sealed class CodeBuild : BuildServerBase
+    {
+        public const string HeadRefEnvironmentName = "CODEBUILD_WEBHOOK_HEAD_REF";
+        private readonly string propertiesFileName;
+
+        public CodeBuild(IEnvironment environment, string propertiesFileName = "gitversion.properties") : base(environment)
+        {
+            this.propertiesFileName = propertiesFileName;
+        }
+
+        protected override string EnvironmentVariable { get; } = HeadRefEnvironmentName;
+
+        public override string GenerateSetVersionMessage(VersionVariables variables)
+        {
+            return variables.FullSemVer;
+        }
+
+        public override string[] GenerateSetParameterMessage(string name, string value)
+        {
+            return new[]
+            {
+                $"GitVersion_{name}={value}"
+            };
+        }
+
+        public override string GetCurrentBranch(bool usingDynamicRepos)
+        {
+            return Environment.GetEnvironmentVariable(HeadRefEnvironmentName);
+        }
+
+        public override void WriteIntegration(Action<string> writer, VersionVariables variables)
+        {
+            base.WriteIntegration(writer, variables);
+            writer($"Outputting variables to '{propertiesFileName}' ... ");
+            File.WriteAllLines(propertiesFileName, BuildOutputFormatter.GenerateBuildLogOutput(this, variables));
+        }
+
+        public override bool PreventFetch() => true;
+    }
+}