Return None from submodules.get() for a nested plain repository

libgit2's git_submodule_lookup reports GIT_EEXISTS, not GIT_ENOTFOUND, when
a repository exists at the path but was never registered as a submodule.
check_error turns that into AlreadyExistsError, which get() did not catch, so
it raised instead of returning None as its docstring promises. __contains__
is built on get(), so `name in repo.submodules` raised for the same input.

__getitem__ keeps raising AlreadyExistsError, so callers that need to tell
"a repository is there" from "nothing is there" still can.

Fixes #1405
Assisted-by: Claude Code (Claude Opus 5)

Author: rawsun007

pygit2/submodules.py

@@ -32,7 +32,7 @@
 from ._pygit2 import Oid
 from .callbacks import RemoteCallbacks, git_fetch_options
 from .enums import SubmoduleIgnore, SubmoduleStatus
-from .errors import check_error
+from .errors import AlreadyExistsError, check_error
 from .ffi import C, ffi
 from .utils import decode_fs_path, decode_string, encode_string
 
@@ -210,7 +210,11 @@ def get(self, name: str) -> Submodule | None:
         """
         try:
             return self[name]
-        except KeyError:
+        except (KeyError, AlreadyExistsError):
+            # libgit2 reports GIT_EEXISTS, which check_error turns into
+            # AlreadyExistsError, when a repository exists at the path but was
+            # never registered as a submodule. There is still no submodule by
+            # that name, so report it the same way as a missing one.
             return None
 
     def add(

test/test_submodule.py

@@ -68,6 +68,25 @@ def test_lookup_missing_submodule(repo: Repository) -> None:
     assert repo.submodules.get('does-not-exist') is None
 
 
+def test_lookup_nested_repo_that_is_not_a_submodule(tmp_path: Path) -> None:
+    """A plain repository inside another is not a submodule.
+
+    libgit2 reports GIT_EEXISTS for this case rather than GIT_ENOTFOUND, which
+    reaches Python as AlreadyExistsError. get() and __contains__ must still
+    describe it as absent, per their documented contracts.
+    """
+    outer = pygit2.init_repository(tmp_path / 'outer')
+    pygit2.init_repository(tmp_path / 'outer' / 'nested')
+
+    assert outer.submodules.get('nested') is None
+    assert 'nested' not in outer.submodules
+
+    # __getitem__ keeps reporting the distinction, so callers that care can
+    # still tell "there is a repository there" from "there is nothing there".
+    with pytest.raises(pygit2.AlreadyExistsError):
+        outer.submodules['nested']
+
+
 def test_listall_submodules(repo: Repository) -> None:
     submodules = repo.listall_submodules()
     assert len(submodules) == 1