Merge pull request #18 from readmeio/merge-libgit-191

- upgrade libgit2 from 1.8.4 to 1.9.1 (via the nodegit forked implementation)
    - [release log here](https://github.com/libgit2/libgit2/releases)
- remove the zlib patches that were necessary in #14
    - the updated libgit2 makes these no longer required to build


Maintenance note: I chose to bring in all the upstream history, with the intent to merge it without squashing so that git knows where we are relative to upstream. This ought to make it easier for me to merge upstream in the future

This PR will bring in a bunch of commits, but only actually merges the diff between https://github.com/nodegit/nodegit/tree/51650c7b4b7d65b35c80d4c62ead63072c54fbcc and https://github.com/nodegit/nodegit/tree/b9f3d16b61762e2ab24b9af10b821468ffe257b0 (view the [diff here](https://github.com/nodegit/nodegit/compare/51650c7..b9f3d16))

Author: llimllib

generate/input/descriptor.json

@@ -2602,9 +2602,11 @@
           "ignore": true
         },
         "git_odb_read": {
+          "isAsync": true,
           "cppFunctionName": "OdbRead",
           "args": {
-            "out": {
+            "obj": {
+              "isReturn": true,
               "ownedByThis": true
             }
           }