git_buf: dispose instead of free

libgit2 has updated some function signatures for consistency, including
`git_buf_free` which is now `git_buf_dispose`.

Author: ethomson

LibGit2Sharp/Core/GitBuf.cs

@@ -12,7 +12,7 @@ internal class GitBuf : IDisposable
 
         public void Dispose()
         {
-            Proxy.git_buf_free(this);
+            Proxy.git_buf_dispose(this);
         }
     }
 }

LibGit2Sharp/Core/NativeMethods.cs

@@ -279,7 +279,7 @@ internal static extern unsafe int git_branch_upstream_name(
             [MarshalAs(UnmanagedType.CustomMarshaler, MarshalCookie = UniqueId.UniqueIdentifier, MarshalTypeRef = typeof(StrictUtf8Marshaler))] string referenceName);
 
         [DllImport(libgit2, CallingConvention = CallingConvention.Cdecl)]
-        internal static extern void git_buf_free(GitBuf buf);
+        internal static extern void git_buf_dispose(GitBuf buf);
 
         [DllImport(libgit2, CallingConvention = CallingConvention.Cdecl)]
         internal static extern unsafe int git_checkout_tree(

LibGit2Sharp/Core/Proxy.cs

@@ -286,9 +286,9 @@ public static unsafe string git_branch_upstream_name(RepositoryHandle handle, st
 
         #region git_buf_
 
-        public static void git_buf_free(GitBuf buf)
+        public static void git_buf_dispose(GitBuf buf)
         {
-            NativeMethods.git_buf_free(buf);
+            NativeMethods.git_buf_dispose(buf);
         }
 
         #endregion