Thanks for all your great work with this plugin; not sure how I'd live without it!

Around ~1% of jobs triggered by GitHub pull requests fail to find the merge commit with output like the below. I'd love to help you debug this as it's been driving me mad for a wee while (I don't know why I've only just filed this now). If I immediately rebuild the job then it always works fine so it's a weird intermittent issue. Even a workaround that would let me e.g. retry the Git operations here would be fantastic.

Thanks folks!

Started by upstream project "Homebrew Pull Requests" build number 19312
originally caused by:
GitHub pull request #35564 of commit 13c8c95db0d399cd387ec362e35d9917df1465f8 automatically merged.
[EnvInject] - Loading node environment variables.
Building remotely on yosemite in workspace /Users/brew/Jenkins/workspace/Homebrew Pull Requests/version/yosemite
> git rev-parse --is-inside-work-tree # timeout=10
Fetching changes from the remote Git repository
> git config remote.origin.url https://github.com/Homebrew/homebrew.git # timeout=10
Fetching upstream changes from https://github.com/Homebrew/homebrew.git
> git --version # timeout=10
> git -c core.askpass=true fetch --tags --progress https://github.com/Homebrew/homebrew.git +refs/heads/:refs/remotes/origin/ +refs/pull/:refs/remotes/origin/pr/ # timeout=5
Checking out Revision c1d392f8a050cb7049b3afee8772b34491150e21 (origin/pr/35564/merge)
> git config core.sparsecheckout # timeout=10
> git checkout -f c1d392f8a050cb7049b3afee8772b34491150e21
FATAL: Could not checkout null with start point c1d392f8a050cb7049b3afee8772b34491150e21
hudson.plugins.git.GitException: Could not checkout null with start point c1d392f8a050cb7049b3afee8772b34491150e21
at org.jenkinsci.plugins.gitclient.CliGitAPIImpl$8.execute(CliGitAPIImpl.java:1674)
at org.jenkinsci.plugins.gitclient.RemoteGitImpl$CommandInvocationHandler$1.call(RemoteGitImpl.java:152)
at org.jenkinsci.plugins.gitclient.RemoteGitImpl$CommandInvocationHandler$1.call(RemoteGitImpl.java:145)
at hudson.remoting.UserRequest.perform(UserRequest.java:121)
at hudson.remoting.UserRequest.perform(UserRequest.java:49)
at hudson.remoting.Request$2.run(Request.java:324)
at hudson.remoting.InterceptingExecutorService$1.call(InterceptingExecutorService.java:68)
at java.util.concurrent.FutureTask.run(FutureTask.java:266)
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617)
at java.lang.Thread.run(Thread.java:745)
at ......remote call to yosemite(Native Method)
at hudson.remoting.Channel.attachCallSiteStackTrace(Channel.java:1356)
at hudson.remoting.UserResponse.retrieve(UserRequest.java:221)
at hudson.remoting.Channel.call(Channel.java:752)
at org.jenkinsci.plugins.gitclient.RemoteGitImpl$CommandInvocationHandler.execute(RemoteGitImpl.java:145)
at sun.reflect.GeneratedMethodAccessor100.invoke(Unknown Source)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:606)
at org.jenkinsci.plugins.gitclient.RemoteGitImpl$CommandInvocationHandler.invoke(RemoteGitImpl.java:131)
at com.sun.proxy.$Proxy52.execute(Unknown Source)
at hudson.plugins.git.GitSCM.checkout(GitSCM.java:992)
at hudson.scm.SCM.checkout(SCM.java:488)
at hudson.model.AbstractProject.checkout(AbstractProject.java:1257)
at hudson.model.AbstractBuild$AbstractBuildExecution.defaultCheckout(AbstractBuild.java:622)
at jenkins.scm.SCMCheckoutStrategy.checkout(SCMCheckoutStrategy.java:86)
at hudson.model.AbstractBuild$AbstractBuildExecution.run(AbstractBuild.java:528)
at hudson.model.Run.execute(Run.java:1745)
at hudson.matrix.MatrixRun.run(MatrixRun.java:146)
at hudson.model.ResourceController.execute(ResourceController.java:89)
at hudson.model.Executor.run(Executor.java:240)
Caused by: hudson.plugins.git.GitException: Command "git checkout -f c1d392f8a050cb7049b3afee8772b34491150e21" returned status code 128:
stdout:
stderr: fatal: reference is not a tree: c1d392f8a050cb7049b3afee8772b34491150e21

at org.jenkinsci.plugins.gitclient.CliGitAPIImpl.launchCommandIn(CliGitAPIImpl.java:1444)
at org.jenkinsci.plugins.gitclient.CliGitAPIImpl.access$500(CliGitAPIImpl.java:85)
at org.jenkinsci.plugins.gitclient.CliGitAPIImpl$8.execute(CliGitAPIImpl.java:1669)
at org.jenkinsci.plugins.gitclient.RemoteGitImpl$CommandInvocationHandler$1.call(RemoteGitImpl.java:152)
at org.jenkinsci.plugins.gitclient.RemoteGitImpl$CommandInvocationHandler$1.call(RemoteGitImpl.java:145)
at hudson.remoting.UserRequest.perform(UserRequest.java:121)
at hudson.remoting.UserRequest.perform(UserRequest.java:49)
at hudson.remoting.Request$2.run(Request.java:324)
at hudson.remoting.InterceptingExecutorService$1.call(InterceptingExecutorService.java:68)
at java.util.concurrent.FutureTask.run(FutureTask.java:266)
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617)
at java.lang.Thread.run(Thread.java:745)

Also viewable at http://bot.brew.sh/job/Homebrew%20Pull%20Requests/19312/version=yosemite/console