PR builds fails on second Appveyor build, for merge-commit

[reisenberger]

Summary:

A PR typically builds twice on AppVeyor - for the branch and then the merge-commit of the PR. The second build often fails with:

========================================
__UpdateAppVeyorBuildNumber
========================================
Executing task: __UpdateAppVeyorBuildNumber
Build with specified version already exists.

The cake build script intends to set the build number from the git metadata FullSemVer and this repeats for the second build.

More analysis at: Polly-Contrib/Simmy#56

Expected behavior:

Both the branch-build and the merge-commit-build of a PR pass.

Actual behaviour:

The merge-commit-build of a PR often fails with above error, even if branch build has passed.

Steps / Code to reproduce the problem:

Make a PR against any non-master branch, without changing gitversionconfig.yml

[reisenberger]

Fixed by #738

[martincostello]

This is still happening.

[martincostello]

I think #921 resolved this once and for all.