Update libgit2 to 0.26

tiennou · tiennou · commit e605d96d6f95 · 2017-11-26T13:04:32.000+01:00
Really, sometimes after 0.26, because of a test .gitmodules which would make Travis fail.
diff --git a/External/libgit2 b/External/libgit2
@@ -1 +1 @@
-Subproject commit 2fcb8705e584ca61f6c4657525c9d2713f6a39d2
+Subproject commit 4f4bc5732baf4545ab42bbd80d2a971e2f1e316c
