Add ProxyOptions for push operations (#872)

Analog to #623 but for push operations rather than fetch.

(cherry picked from commit 5eca48c)

remote.go

@@ -291,6 +291,9 @@ type PushOptions struct {
 
  // Headers are extra headers for the push operation.
  Headers []string
+
+ // Proxy options to use for this push operation
+ ProxyOptions ProxyOptions
 }
 
 type RemoteHead struct {
@@ -991,6 +994,7 @@ func populatePushOptions(copts *C.git_push_options, opts *PushOptions, errorTarg
  strings: makeCStringsFromStrings(opts.Headers),
  }
  populateRemoteCallbacks(&copts.callbacks, &opts.RemoteCallbacks, errorTarget)
+ populateProxyOptions(&copts.proxy_opts, &opts.ProxyOptions)
  return copts
 }
 
@@ -1000,6 +1004,7 @@ func freePushOptions(copts *C.git_push_options) {
  }
  untrackCallbacksPayload(&copts.callbacks)
  freeStrarray(&copts.custom_headers)
+ freeProxyOptions(&copts.proxy_opts)
 }
 
 // Fetch performs a fetch operation. refspecs specifies which refspecs