lib: improve API unstability note
xs: update libgit2
v0.20

 - Documentation fixes (GH#6) (thanks, @daknok!)
 - Add note in the documentation about the API being unstable
 - Update libgit2 to f42beff
    + Update checkout strategies' names: "update_unmodified", "update_missing",
      "update_modified" and "update_untracked" have been removed (incompatible
      change)
 - Repository -> head() now takes an additional optional parameter $new_head
   used to change the repository's HEAD (GH#7)
 - Repository -> workdir() now takes an additional optional parameter $new_dir
   used to change the repository's working directory
   (GH#8, #9) (thanks, @thaljef)
 - Add Tree -> entry_by{name,path}() methods (GH#10) (thanks, @keedi!)