jenkins skip right last commit in gitlab merge request webhook

## Issue

### Context
- **Gitlab plugin version**: 1.5.13
- **Gitlab version**: 11.11.8
- **Jenkins version**: 2.176.1
- **Job type**: Pipeline

### Problem description
I create a job with gitlab plugin in jenkins, and create webhook for multi gitlab repository's(we call A and B) merge request.

The Step:

1. Create a merge request for A, it will auto trigger jenkins job and Finished as SUCCESS
2. Create a merge request for B, it will auto trigger jenkins job and Finished as SUCCESS
3. trigger jenkins job from A/hooks/{id}/edit webhook page of gitlab, the job fail.

After then, trigger jenkins job from webhook page of gitlab for repository B will always SUCCESS, But for repository A(even if i close the origin merge request and recreate it), it always FAILED.

From the Job console log. we can see github plugin can't get the **right last commit** and do a right fetch.
As below the commit id `07c1de9abcffb4e70588bc0d5ec6a5230dfaf89f` is the last commit id of repository B，and the commit id `09f1de6b5b4964b4ed092525b29265e799d7e594` is the last commit id of repository A.

job console log for repository A
```
using GIT_ASKPASS to set credentials ****.com
> git fetch --tags --progress http://A.git +refs/heads/*:refs/remotes/origin/* +refs/merge-requests/*/head:refs/remotes/origin/merge-requests/*
skipping resolution of commit 09f1de6b5b4964b4ed092525b29265e799d7e594, since it originates from another repository
Seen branch in repository origin/fix_pep8
Seen branch in repository origin/master
Seen branch in repository origin/merge-requests/1
Seen branch in repository origin/merge-requests/2
Seen branch in repository origin/merge-requests/3
Seen branch in repository origin/merge-requests/4
Seen branch in repository origin/merge-requests/5
Seen branch in repository origin/merge-requests/6
Seen branch in repository origin/merge-requests/7
Seen branch in repository origin/merge-requests/8
Seen branch in repository origin/o-develop
Seen branch in repository origin/r-develop
Seen branch in repository origin/stable/o
Seen branch in repository origin/stable/r
Seen branch in repository origin/1.4-release
Seen branch in repository origin/1.4.0-release
Seen 16 remote branches
> git show-ref --tags -d # timeout=10
Checking out Revision 07c1de9abcffb4e70588bc0d5ec6a5230dfaf89f (origin/fix_pep8, origin/merge-requests/2, origin/o-develop)
> git config core.sparsecheckout # timeout=10
> git checkout -f 07c1de9abcffb4e70588bc0d5ec6a5230dfaf89f
FATAL: Could not checkout 07c1de9abcffb4e70588bc0d5ec6a5230dfaf89f
hudson.plugins.git.GitException: Command "git checkout -f 07c1de9abcffb4e70588bc0d5ec6a5230dfaf89f" returned status code 128:
stdout:
stderr: fatal: 引用不是一个树：07c1de9abcffb4e70588bc0d5ec6a5230dfaf89f

at org.jenkinsci.plugins.gitclient.CliGitAPIImpl.launchCommandIn(CliGitAPIImpl.java:2042)
at org.jenkinsci.plugins.gitclient.CliGitAPIImpl.access$900(CliGitAPIImpl.java:72)
at org.jenkinsci.plugins.gitclient.CliGitAPIImpl$9.execute(CliGitAPIImpl.java:2341)
Caused: hudson.plugins.git.GitException: Could not checkout 07c1de9abcffb4e70588bc0d5ec6a5230dfaf89f
at org.jenkinsci.plugins.gitclient.CliGitAPIImpl$9.execute(CliGitAPIImpl.java:2365)
at hudson.plugins.git.GitSCM.checkout(GitSCM.java:1242)
at hudson.scm.SCM.checkout(SCM.java:504)
at hudson.model.AbstractProject.checkout(AbstractProject.java:1208)
at hudson.model.AbstractBuild$AbstractBuildExecution.defaultCheckout(AbstractBuild.java:574)
at jenkins.scm.SCMCheckoutStrategy.checkout(SCMCheckoutStrategy.java:86)
at hudson.model.AbstractBuild$AbstractBuildExecution.run(AbstractBuild.java:499)
at hudson.model.Run.execute(Run.java:1818)
at hudson.model.FreeStyleBuild.run(FreeStyleBuild.java:43)
at hudson.model.ResourceController.execute(ResourceController.java:97)
at hudson.model.Executor.run(Executor.java:429)
Finished: FAILURE
```