include: Unify internal include strategies

Do not add the `git2` path to internal includes, or that will cause
an extra path dependency.

include/git2/indexer.h

@@ -7,8 +7,8 @@
 #ifndef _INCLUDE_git_indexer_h__
 #define _INCLUDE_git_indexer_h__
 
-#include "git2/common.h"
-#include "git2/oid.h"
+#include "common.h"
+#include "oid.h"
 
 GIT_BEGIN_DECL
 

include/git2/refspec.h

@@ -7,7 +7,7 @@
 #ifndef INCLUDE_git_refspec_h__
 #define INCLUDE_git_refspec_h__
 
-#include "git2/types.h"
+#include "types.h"
 
 /**
  * @file git2/refspec.h

include/git2/remote.h

@@ -7,9 +7,9 @@
 #ifndef INCLUDE_git_remote_h__
 #define INCLUDE_git_remote_h__
 
-#include "git2/common.h"
-#include "git2/repository.h"
-#include "git2/refspec.h"
+#include "common.h"
+#include "repository.h"
+#include "refspec.h"
 /**
  * @file git2/remote.h
  * @brief Git remote management functions