Update libgit2 to 821131f

The API changes are:

- `*Remote.Connect`

  ```go
  // from:
  func (o *Remote) Connect(direction ConnectDirection, callbacks *RemoteCallbacks) error
  // to:
  func (o *Remote) Connect(direction ConnectDirection, callbacks *RemoteCallbacks, headers []string) error
  ```

- `*Remote.ConnectFetch` - `headers` was added as above
- `*Remote.ConnectPush` - `headers` was added as above

1 files changed, 20 insertions, 2 deletions

diff --git a/git.go b/git.go
index 34d58e6..2b2a909 100644
--- a/git.go
+++ b/git.go
@@ -52,6 +52,7 @@ const (
 
 	// No error
 	ErrOk ErrorCode = C.GIT_OK
+
 	// Generic error
 	ErrGeneric ErrorCode = C.GIT_ERROR
 	// Requested object could not be found
@@ -62,10 +63,12 @@ const (
 	ErrAmbigious ErrorCode = C.GIT_EAMBIGUOUS
 	// Output buffer too short to hold data
 	ErrBuffs ErrorCode = C.GIT_EBUFS
+
 	// GIT_EUSER is a special error that is never generated by libgit2
 	// code.  You can return it from a callback (e.g to stop an iteration)
 	// to know that it was generated by the callback and not by libgit2.
 	ErrUser ErrorCode = C.GIT_EUSER
+
 	// Operation not allowed on bare repository
 	ErrBareRepo ErrorCode = C.GIT_EBAREREPO
 	// HEAD refers to branch with no commits
@@ -82,12 +85,27 @@ const (
 	ErrLocked ErrorCode = C.GIT_ELOCKED
 	// Reference value does not match expected
 	ErrModified ErrorCode = C.GIT_EMODIFIED
+	// Authentication failed
+	ErrAuth ErrorCode = C.GIT_EAUTH
+	// Server certificate is invalid
+	ErrCertificate ErrorCode = C.GIT_ECERTIFICATE
+	// Patch/merge has already been applied
+	ErrApplied ErrorCode = C.GIT_EAPPLIED
+	// The requested peel operation is not possible
+	ErrPeel ErrorCode = C.GIT_EPEEL
+	// Unexpected EOF
+	ErrEOF ErrorCode = C.GIT_EEOF
+	// Uncommitted changes in index prevented operation
+	ErrUncommitted ErrorCode = C.GIT_EUNCOMMITTED
+	// The operation is not valid for a directory
+	ErrDirectory ErrorCode = C.GIT_EDIRECTORY
+	// A merge conflict exists and cannot continue
+	ErrMergeConflict ErrorCode = C.GIT_EMERGECONFLICT
+
 	// Internal only
 	ErrPassthrough ErrorCode = C.GIT_PASSTHROUGH
 	// Signals end of iteration with iterator
 	ErrIterOver ErrorCode = C.GIT_ITEROVER
-	// Authentication failed
-	ErrAuth ErrorCode = C.GIT_EAUTH
 )
 
 var (