Honor clone depth property from pipeline on checkout

[cmur2]

When using the SSH runner from DockerHub drone/drone-runner-ssh:latest with a pipeline like:

kind: pipeline
type: ssh
name: ci

server:
  host: XXX
  user: XXX
  ssh_key:
    from_secret: XXX

clone:
  depth: 1

steps:
- name: hello
  commands:
  - echo Hello

...the output shows that the depth: 1 property is not respected although the docs say so:

+ git init
Initialized empty Git repository in /tmp/drone-DNfuPrrazu5C3MMX/drone/src/.git/
+ git remote add origin XXX
+ git fetch  origin +refs/heads/add-ci:
From XXX
 * branch            add-ci     -> FETCH_HEAD
 * [new branch]      add-ci     -> origin/add-ci
+ git checkout XXX -b add-ci
Switched to a new branch 'add-ci'

Looking at the source code the relevant code seems to be at

drone-runner-ssh/engine/compiler/compiler.go

Lines 207 to 212 in 8d2ec31

	clone.Args{
	Branch: c.Build.Target,
	Commit: c.Build.After,
	Ref: c.Build.Ref,
	Remote: c.Repo.HTTPURL,
	},

not passing the information.

This PR addresses that problem and I already verified with a locally built Docker image that it works: + git fetch --depth=1 origin +refs/heads/add-ci:

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[cmur2]

@bradrydzewski could you take a look at this PR and also #3 ?