Add option to disable the complete GitVersion.Task for building with no existing repositories

[DerAlbertCom]

Hi,

since v5.0.0-beta-3-27 it is not possible to use a GitVersion with tools like NCrunch, which copies the source files for building and don't have a copy of the git repository.

@onovotny fixed this in #1193 very NCrunch Specific, these changes are not in the current version anymore. Maybe because of NCrunch is too specific.

Now i must disable the GitVersion.Task with something like

  <PropertyGroup Condition="'$(NCrunch)' == '1'">
    <WriteVersionInfoToBuildLog>false</WriteVersionInfoToBuildLog>
    <UpdateAssemblyInfo>false</UpdateAssemblyInfo>
    <GenerateGitVersionInformation>false</GenerateGitVersionInformation>
    <GetVersion>false</GetVersion>
    <GenerateGitVersionWixDefines>false</GenerateGitVersionWixDefines>
    <UpdateVersionProperties>false</UpdateVersionProperties>
    <UseFullSemVerForNuGet>false</UseFullSemVerForNuGet>
  </PropertyGroup>

which is plenty of stuff, and may change over time.

So I propose a general off switch. So i can do something like.

  <PropertyGroup Condition="'$(NCrunch)' == '1'">
    <DisableGitVersionTask>true</DisableGitVersionTask>
  </PropertyGroup>

I willing to provide a PR, but would like to check the naming, or if there is a strong "we will never do that" within the team.

[arturcic]

I guess it makes sense to have such a condition. Please submit a PR and then we can come up with a name.
DisableGitVersionTask seems like a good name.

[arturcic]

implemented in 04818f2