Provide user friendly error for when PR is merged before action runs

[jsoref]

#36 reports users can be confused by:

couldn't find remote ref refs/pull/6/merge

An action run output will look like:

 Check failure on line 1 in .github

@github-actionsgithub-actions
/ changes
.github#L1
couldn't find remote ref refs/pull/6/merge
 Check warning on line 1 in .github

@github-actionsgithub-actions
/ changes
.github#L1
Git fetch failed with exit code 128, back off 7.292 seconds before retry.
 Check failure on line 1 in .github

@github-actionsgithub-actions
/ changes
.github#L1
couldn't find remote ref refs/pull/6/merge
 Check warning on line 1 in .github

@github-actionsgithub-actions
/ changes
.github#L1
Git fetch failed with exit code 128, back off 8.111 seconds before retry.
 Check failure on line 1 in .github

@github-actionsgithub-actions
/ changes
.github#L1
couldn't find remote ref refs/pull/6/merge
 Check failure on line 1 in .github

@github-actionsgithub-actions
/ changes
.github#L1
Git fetch failed with exit code: 128
 Check failure on line 1 in .github

@github-actionsgithub-actions
/ changes
.github#L1
Exit code 1 returned from process: file name '/home/runner/runners/2.274.2/bin/Runner.PluginHost', arguments 'action "GitHub.Runner.Plugins.Repository.v1_0.CheckoutTask, Runner.Plugins"'.

While it's technically correct, it'd be much nicer if the error message was replaced with a friendly message.

Something like Can't retrieve merge, PR has already been merged or Can't retrieve merge, PR has been closed.

I'm hitting this often and it's pretty frustrating. Technically correct, but...

Also, maybe it's worth mapping this to skipped instead of failed.

[jsoref]

Also, apparently sometimes as in PowerDNS/pdns#10147, one can hit this w/o the PR being closed, so it'd be really nice if there was some debugging to explain what's going on. We're left scratching our heads.

[jsoref]

I think that the initial description included output from a problem-matcher, here's current log output:

Log view

✅Set up job 3s
❌checkout-merge 27s
▶️Run actions/checkout@v2
Syncing repository: jsoref/checkout-merge
▶️Getting Git version info
Deleting the contents of '/home/runner/work/checkout-merge/checkout-merge'
▶️Initializing the repository
▶️Disabling automatic garbage collection
▶️Setting up auth
▼Fetching the repository
  /usr/bin/git -c protocol.version=2 fetch --no-tags --prune --progress --no-recurse-submodules --depth=1 origin +refs/pull/1/merge:refs/remotes/pull/1/merge
  Error: fatal: couldn't find remote ref refs/pull/1/merge
  The process '/usr/bin/git' failed with exit code 128
  Waiting 13 seconds before trying again
  /usr/bin/git -c protocol.version=2 fetch --no-tags --prune --progress --no-recurse-submodules --depth=1 origin +refs/pull/1/merge:refs/remotes/pull/1/merge
  Error: fatal: couldn't find remote ref refs/pull/1/merge
  The process '/usr/bin/git' failed with exit code 128
  Waiting 13 seconds before trying again
  /usr/bin/git -c protocol.version=2 fetch --no-tags --prune --progress --no-recurse-submodules --depth=1 origin +refs/pull/1/merge:refs/remotes/pull/1/merge
  Error: fatal: couldn't find remote ref refs/pull/1/merge
  Error: The process '/usr/bin/git' failed with exit code 128
🚫checkout 0s
🚫check-count 0s