packp/capability: Skip argument validations for unknown capabilities

[orirawlings]

Fixes #623

[codecov]

Codecov Report

Merging #626 into master will decrease coverage by 0.56%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #626      +/-   ##
==========================================
- Coverage   78.26%   77.69%   -0.57%     
==========================================
  Files         130      130              
  Lines       10197    10201       +4     
==========================================
- Hits         7981     7926      -55     
- Misses       1356     1428      +72     
+ Partials      860      847      -13

Impacted Files	Coverage Δ
plumbing/protocol/packp/capability/capability.go	100% <ø> (ø)	⬆️
plumbing/protocol/packp/capability/list.go	97.53% <100%> (+0.12%)	⬆️
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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[dsvensson]

Issue creator here... this is the output before vs after this bug fix (tl;dr fix works as expected):

$ cat foo.go
package main

import (
    "fmt"
    git "gopkg.in/src-d/go-git.v4"
)

func main() {
    r, err := git.PlainClone("output", false, &git.CloneOptions{
        URL:               "https://gopkg.in/yaml.v2",
        RecurseSubmodules: git.DefaultSubmoduleRecursionDepth,
    })
    fmt.Println(err)
    fmt.Println(r)
}
go run foo.go
pkt-line 3: invalid capabilities: arguments not allowed (symref=HEAD:refs/heads/v2 multi_ack thin-pack side-band side-band-64k ofs-delta shallow deepen-since deepen-not deepen-relative no-progress include-tag multi_ack_detailed no-done oldref=HEAD:refs/heads/v2 agent=git/github-g09a9ed6d3a)
&{0xc4200683c0 map[] 0xc420121620}
$ ls output
$ find output
output
output/.git
output/.git/config
output/.git/HEAD
output/.git/objects
output/.git/objects/info
output/.git/objects/pack
output/.git/refs
output/.git/refs/heads
output/.git/refs/tags
$ cat output/.git/config
[core]
bare = false
[remote "origin"]
url = https://gopkg.in/yaml.v2
fetch = +refs/heads/*:refs/remotes/origin/*

$ mv vendor/gopkg.in/src-d/go-git.v4 vendor/gopkg.in/src-d/go-git.v4.orig
$ ln -sf ../../../go-git-fixed vendor/gopkg.in/src-d/go-git.v4
$ rm -rf output
$ go run foo.go
<nil>
&{0xc4200683c0 map[] 0xc42012b640}
$ find output
output
output/.git
output/.git/config
output/.git/HEAD
output/.git/index
output/.git/objects
output/.git/objects/info
output/.git/objects/pack
output/.git/objects/pack/pack-fc51ec5c6df85d7b5bc63a332f75e089ecccbff7.idx
output/.git/objects/pack/pack-fc51ec5c6df85d7b5bc63a332f75e089ecccbff7.pack
output/.git/refs
output/.git/refs/heads
output/.git/refs/heads/v2
output/.git/refs/remotes
output/.git/refs/remotes/origin
output/.git/refs/remotes/origin/master
output/.git/refs/remotes/origin/rename-imports
output/.git/refs/remotes/origin/v1
output/.git/refs/remotes/origin/v2
output/.git/refs/tags
output/.travis.yml
output/apic.go
output/decode.go
output/decode_test.go
output/emitterc.go
output/encode.go
output/encode_test.go
output/example_embedded_test.go
output/LICENSE
output/LICENSE.libyaml
output/parserc.go
output/readerc.go
output/README.md
output/resolve.go
output/scannerc.go
output/sorter.go
output/suite_test.go
output/writerc.go
output/yaml.go
output/yamlh.go
output/yamlprivateh.go