Add sideband support for push

[orirawlings]

Fixes #528

An example of using the code to push to a local repository that always fails in a pre-receive hook (but pre-receive hook outputs some ascii art first):

$ go run main.go . local +refs/tags/initial-not-annotated:refs/heads/pushed
                                                                                 
                                                                                 
NNNNNNNN        NNNNNNNN                                                         
N:::::::N       N::::::N                                                         
N::::::::N      N::::::N                                                         
N:::::::::N     N::::::N                                                         
N::::::::::N    N::::::N   ooooooooooo   ppppp   ppppppppp       eeeeeeeeeeee    
N:::::::::::N   N::::::N oo:::::::::::oo p::::ppp:::::::::p    ee::::::::::::ee  
N:::::::N::::N  N::::::No:::::::::::::::op:::::::::::::::::p  e::::::eeeee:::::ee
N::::::N N::::N N::::::No:::::ooooo:::::opp::::::ppppp::::::pe::::::e     e:::::e
N::::::N  N::::N:::::::No::::o     o::::o p:::::p     p:::::pe:::::::eeeee::::::e
N::::::N   N:::::::::::No::::o     o::::o p:::::p     p:::::pe:::::::::::::::::e 
N::::::N    N::::::::::No::::o     o::::o p:::::p     p:::::pe::::::eeeeeeeeeee  
N::::::N     N:::::::::No::::o     o::::o p:::::p    p::::::pe:::::::e           
N::::::N      N::::::::No:::::ooooo:::::o p:::::ppppp:::::::pe::::::::e          
N::::::N       N:::::::No:::::::::::::::o p::::::::::::::::p  e::::::::eeeeeeee  
N::::::N        N::::::N oo:::::::::::oo  p::::::::::::::pp    ee:::::::::::::e  
NNNNNNNN         NNNNNNN   ooooooooooo    p::::::pppppppp        eeeeeeeeeeeeee  
                                          p:::::p                                
                                          p:::::p                                
                                         p:::::::p                               
                                         p:::::::p                               
                                         p:::::::p                               
                                         ppppppppp                               
                                                                                 
error: command error on refs/heads/pushed: pre-receive hook declined
exit status 1

Here is another example that prints the sideband data sent by github when trying to force push to a protected branch:

$ go run main.go . github +refs/tags/initial-not-annotated:refs/heads/pushed
error: GH006: Protected branch update failed for refs/heads/pushed.
error: Cannot force-push to a protected branch
error: command error on refs/heads/pushed: protected branch hook declined
exit status 1

The main.go program I used for these examples can be found below:

package main

import (
	"os"

	"gopkg.in/src-d/go-git.v4"
	. "gopkg.in/src-d/go-git.v4/_examples"
	"gopkg.in/src-d/go-git.v4/config"
)

// Example of a push with non-default options and progress reporting
func main() {
	CheckArgs("<repository-path>", "<remote>", "<refspec> [<refspec> ...]")
	path := os.Args[1]
	remote := os.Args[2]

	var refSpecs []config.RefSpec
	for _, s := range os.Args[3:] {
		refSpecs = append(refSpecs, config.RefSpec(s))
	}

	r, err := git.PlainOpen(path)
	CheckIfError(err)

	err = r.Push(&git.PushOptions{
		RemoteName: remote,
		RefSpecs:   refSpecs,
		Progress:   os.Stdout,
	})
	CheckIfError(err)
}

[codecov]

Codecov Report

Merging #573 into master will decrease coverage by 0.64%.
The diff coverage is 57.14%.

@@            Coverage Diff             @@
##           master     #573      +/-   ##
==========================================
- Coverage   78.01%   77.36%   -0.65%     
==========================================
  Files         129      129              
  Lines        9855     9874      +19     
==========================================
- Hits         7688     7639      -49     
- Misses       1328     1405      +77     
+ Partials      839      830       -9

Impacted Files	Coverage Δ
plumbing/protocol/packp/updreq.go	80% <ø> (ø)	⬆️
options.go	84.48% <ø> (ø)	⬆️
plumbing/transport/http/receive_pack.go	70% <12.5%> (-10.96%)	⬇️
remote.go	74.25% <84.61%> (+0.21%)	⬆️
plumbing/transport/ssh/common.go	20.54% <0%> (-45.21%)	⬇️
plumbing/transport/ssh/auth_method.go	31.57% <0%> (-22.81%)	⬇️

---

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[mcuadros]

Great! Can you cover it with some test? I know it hard but will be very nice.

[orirawlings]

@mcuadros I've gone ahead and added a test. It relies on some platform specific pre-receive hooks (unlike git-upload-pack, git-receive-pack doesn't send sideband data by default).