Update vendored libgit2 to a released v0.26
diff --git a/vendor/libgit2 b/vendor/libgit2
index df4dfaa..15e1193 160000
--- a/vendor/libgit2
+++ b/vendor/libgit2
@@ -1 +1 @@
-Subproject commit df4dfaadcf709646ebab2e57e3589952cf1ac809
+Subproject commit 15e119375018fba121cf58e02a9f17fe22df0df8