(split from main repo) Add and use IExecutable and IProcess abstractions

This change introduces an interface for the use and substitution of
executables and the processes they create.

GitModule accepts an instance of IExecutable that represents git.exe,
allowing injection of MockExecutable for testing purposes.
See GitModuleTest.

ExecutableExtensions provide convenient access to the most common
operations required from executables.

Notably all executions that read process output now do so using
async/await, rather than spawning one-off threads to read output
streams. This gives better utilisation of the thread pool.

The lightweight type ArgumentString is used as a union type between
string and ArgumentBuilder (and potential future types) for passing
arguments. It has implicit conversions to string, and from both
string and ArgumentBuilder.

All process invocations are now tracked in CommandLog.

CommandCache has been converted from a static class into an instance
class so that it may be used for any execution that returns output,
whether for git.exe or other processes. It has been changed to reuse
the MruCache class that was recently added for in-memory avatar caching.

GitCommandHelpers.VersionInUse has been moved to GitVersion.Current
where it has a clearer relationship.

Author: drewnoakes

FormGerritDownload.cs

@@ -148,7 +148,7 @@ private bool RunCommand(IGitRemoteCommand command, string change)
         private static string FetchCommand(string remote, string remoteBranch)
         {
             var progressOption = "";
-            if (GitCommandHelpers.VersionInUse.FetchCanAskForProgress)
+            if (GitVersion.Current.FetchCanAskForProgress)
             {
                 progressOption = "--progress ";
             }

FormGerritPublish.cs

@@ -40,17 +40,15 @@ private void PublishClick(object sender, EventArgs e)
             }
         }
 
-        private static string PushCmd(string remote, string toBranch)
+        private static ArgumentString PushCmd(string remote, string toBranch)
         {
-            var args = new ArgumentBuilder
+            return new ArgumentBuilder
             {
                 "push",
-                { GitCommandHelpers.VersionInUse.PushCanAskForProgress, "--progress" },
+                { GitVersion.Current.PushCanAskForProgress, "--progress" },
                 remote.ToPosixPath().Trim().Quote(),
                 $"HEAD:{GitRefName.GetFullBranchName(toBranch)?.Replace(" ", "")}"
             };
-
-            return args.ToString();
         }
 
         private bool PublishChange(IWin32Window owner)

GerritUtil.cs

@@ -60,11 +60,9 @@ public static async Task<string> RunGerritCommandAsync([NotNull] IWin32Window ow
 
             StartAgent(owner, module, remote);
 
-            var sshCmd = SshPathLocatorInstance.Find(AppSettings.GitBinDir);
-            if (GitCommandHelpers.Plink())
-            {
-                sshCmd = AppSettings.Plink;
-            }
+            var sshCmd = GitCommandHelpers.Plink()
+                ? AppSettings.Plink
+                : SshPathLocatorInstance.Find(AppSettings.GitBinDir);
 
             if (string.IsNullOrEmpty(sshCmd))
             {
@@ -100,11 +98,8 @@ public static async Task<string> RunGerritCommandAsync([NotNull] IWin32Window ow
             sb.Append(command);
             sb.Append("\"");
 
-            return await module.RunCmdAsync(
-                sshCmd,
-                sb.ToString(),
-                encoding: null,
-                stdIn).ConfigureAwait(false);
+            return await new Executable(sshCmd)
+                .GetOutputAsync(sb.ToString(), stdIn).ConfigureAwait(false);
         }
 
         public static void StartAgent([NotNull] IWin32Window owner, [NotNull] IGitModule module, [NotNull] string remote)