repository: only reserve repo dirs in the workdir

Check that the repository directory is beneath the workdir before
adding it to the list of reserved paths.  If it is not, then there
is no possibility of checking out files into it, and it should not
be a reserved word.

This is a particular problem with submodules where the repo directory
may be in the super's .git directory.

src/repository.c

@@ -908,12 +908,28 @@ bool git_repository__reserved_names(
 			buf->size = git_repository__reserved_names_win32[i].size;
 		}
 
-		/* Try to add any repo-specific reserved names */
+		/* Try to add any repo-specific reserved names - the gitlink file
+		 * within a submodule or the repository (if the repository directory
+		 * is beneath the workdir).  These are typically `.git`, but should
+		 * be protected in case they are not.  Note, repo and workdir paths
+		 * are always prettified to end in `/`, so a prefixcmp is safe.
+		 */
 		if (!repo->is_bare) {
-			const char *reserved_path = repo->path_gitlink ?
-				repo->path_gitlink : repo->path_repository;
+			int (*prefixcmp)(const char *, const char *);
+			int error, ignorecase;
+
+			error = git_repository__cvar(
+				&ignorecase, repo, GIT_CVAR_IGNORECASE);
+			prefixcmp = (error || ignorecase) ? git__prefixcmp_icase :
+				git__prefixcmp;
+
+			if (repo->path_gitlink &&
+				reserved_names_add8dot3(repo, repo->path_gitlink) < 0)
+				goto on_error;
 
-			if (reserved_names_add8dot3(repo, reserved_path) < 0)
+			if (repo->path_repository &&
+				prefixcmp(repo->path_repository, repo->workdir) == 0 &&
+				reserved_names_add8dot3(repo, repo->path_repository) < 0)
 				goto on_error;
 		}
 	}