Make pygit2 throw if tree of a commit is not found

Commit objects in git always have a tree_id associated, that points
to the corresponding Tree object.

When the tree object is missing, the repo is corrupted.

In those cases:
* official git cli fatals with status code 128 and message:
  fatal: unable to read tree <hash>
* libgit2 returns error GIT_ENOTFOUND when calling git_commit_tree()
* pygit2 when accessing the tree by id with repo[commit.tree_id]
raises a KeyError: <hash>

But on the other hand, on the commit object, rather than throwing
and exception, pygit2 is swallowing the error returned by libgit2
and setting the <Commit object>.tree property to None.

None is arguable the wrong choice to encode an error condition,
specially in python that is used heavily.

In particular this caused in our system to assume there was
an empty tree, and the sync service that tails git repo changes
decided to DELETE everything. The code was using None to
represent empty tree, usefull for example when we need to
compare a path between two commits (the path might be non-existant
at one of the commits you are comparing).

I think that in this case the right decision would be to raise
since is an exceptional case, caused by a corrupted repo,
is more consistent with other tools, and ensures user code
does not take the wrong decissions.

For curiosity the corrupted repository can happen more commonly
than expected. We run our repositories on a shared NFS filer,
and one of our servers didn't have the lookupcache=positive
flag. This makes NFS cache the metadata (files on a directory
for example) and use that for negative lookups (to deny existance
of files). In this case, the commit object was on a directory
not cached, so the commit was seen immediately, but the tree
object was in a folder that was cached, the cache didn't
contained the tree object, and thus for some seconds the tree
was not existing and the repo was corrupted. Our sync service
saw tree being None and decided to delete everything, causing
a lot of issues down the way.

Author: Guillermo Pérez

src/commit.c

@@ -35,6 +35,7 @@
 #include "oid.h"
 
 extern PyTypeObject TreeType;
+extern PyObject *GitError;
 
 
 PyDoc_STRVAR(Commit_message_encoding__doc__, "Message encoding.");
@@ -131,8 +132,11 @@ Commit_tree__get__(Commit *commit)
     int err;
 
     err = git_commit_tree(&tree, commit->commit);
-    if (err == GIT_ENOTFOUND)
-        Py_RETURN_NONE;
+    if (err == GIT_ENOTFOUND) {
+        char tree_id[GIT_OID_HEXSZ + 1] = { 0 };
+        git_oid_fmt(tree_id, git_commit_tree_id(commit->commit));
+        return PyErr_Format(GitError, "Unable to read tree %s", tree_id);
+    }
 
     if (err < 0)
         return Error_set(err);