Improve error messages for running git clone against a Mercurial repository

Summary:
Fixes T11938.

Note that there's a subcase here: if you `hg clone` or `svn checkout` a short `/source/` URI that ends in `.git`, we miss the lookup and don't get this far, so you still get a generic error message.

Hopefully it is clear enough on its own that `proto://.../blah.git` is, in fact, a Git repository, since it says ".git" at the end.

If that doesn't prove to be true, we can be more surgical about this.

Test Plan:
```
$ git clone ssh://local@localvault.phacility.com/source/quack.notgit/
Cloning into 'quack.notgit'...
phabricator-ssh-exec: This repository ("quack.notgit") is not a Git repository. Use "hg" to interact with this repository.
fatal: Could not read from remote repository.

Please make sure you have the correct access rights
and the repository exists.
```

```
$ hg clone ssh://local@localvault.phacility.com/source/phabx
remote: phabricator-ssh-exec: This repository ("phabx") is not a Mercurial repository. Use "git" to interact with this repository.
abort: no suitable response from remote hg!
```

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T11938

Differential Revision: https://secure.phabricator.com/D16976

src/applications/diffusion/ssh/DiffusionGitSSHWorkflow.php

@@ -35,4 +35,14 @@ abstract class DiffusionGitSSHWorkflow
     }
   }
 
+  protected function raiseWrongVCSException(
+    PhabricatorRepository $repository) {
+    throw new Exception(
+      pht(
+        'This repository ("%s") is not a Git repository. Use "%s" to '.
+        'interact with this repository.',
+        $repository->getDisplayName(),
+        $repository->getVersionControlSystem()));
+  }
+
 }

src/applications/diffusion/ssh/DiffusionMercurialServeSSHWorkflow.php

@@ -119,4 +119,14 @@ final class DiffusionMercurialServeSSHWorkflow
     return $raw_message;
   }
 
+  protected function raiseWrongVCSException(
+    PhabricatorRepository $repository) {
+    throw new Exception(
+      pht(
+        'This repository ("%s") is not a Mercurial repository. Use "%s" to '.
+        'interact with this repository.',
+        $repository->getDisplayName(),
+        $repository->getVersionControlSystem()));
+  }
+
 }

src/applications/diffusion/ssh/DiffusionSSHWorkflow.php

@@ -43,6 +43,8 @@ abstract class DiffusionSSHWorkflow extends PhabricatorSSHWorkflow {
    */
   abstract protected function identifyRepository();
   abstract protected function executeRepositoryOperations();
+  abstract protected function raiseWrongVCSException(
+    PhabricatorRepository $repository);
 
   protected function getBaseRequestPath() {
     return $this->baseRequestPath;
@@ -199,6 +201,10 @@ abstract class DiffusionSSHWorkflow extends PhabricatorSSHWorkflow {
           $repository->getDisplayName()));
     }
 
+    if ($repository->getVersionControlSystem() != $vcs) {
+      $this->raiseWrongVCSException($repository);
+    }
+
     return $repository;
   }
 

src/applications/diffusion/ssh/DiffusionSubversionServeSSHWorkflow.php

@@ -449,4 +449,14 @@ final class DiffusionSubversionServeSSHWorkflow
     return $path;
   }
 
+  protected function raiseWrongVCSException(
+    PhabricatorRepository $repository) {
+    throw new Exception(
+      pht(
+        'This repository ("%s") is not a Subversion repository. Use "%s" to '.
+        'interact with this repository.',
+        $repository->getDisplayName(),
+        $repository->getVersionControlSystem()));
+  }
+
 }