Tests fail: Can't find git

[asbjornu]

While attempting to add profiling to GitVersion in order to debug #437, I stumbled upon an issue running the following tests:

1. PullRequestScenarios.CanCalculatePullRequestChanges
2. PullRequestScenarios.CanCalculatePullRequestChangesFromRemoteRepo
3. PullRequestScenarios.CanCalculatePullRequestChangesInheritingConfig
4. GitPreparerTests.UpdatesExistingDynamicRepository
5. GitHelperTests.CanDetermineTheVersionFromAFetchedFeature

They are all failing for the same reason: git can't be found. Here's the stack trace:

System.IO.FileNotFoundException : The executable file 'git' could not be found.
  ----> System.ComponentModel.Win32Exception : The system cannot find the file specified
   at GitVersion.Helpers.ProcessHelper.Start(ProcessStartInfo startInfo) in ProcessHelper.cs: line 40
   at GitVersion.Helpers.ProcessHelper.Run(Action`1 output, Action`1 errorOutput, TextReader input, String exe, String args, String workingDirectory, KeyValuePair`2[] environmentalVariables) in ProcessHelper.cs: line 79
   at GitTestExtensions.DumpGraph(IRepository repository) in GitTestExtensions.cs: line 18
   at EmptyRepositoryFixture.DumpGraph() in EmptyRepositoryFixture.cs: line 14
   at PullRequestScenarios.CanCalculatePullRequestChanges() in PullRequestScenarios.cs: line 19
--Win32Exception
   at System.Diagnostics.Process.StartWithCreateProcess(ProcessStartInfo startInfo)
   at System.Diagnostics.Process.Start(ProcessStartInfo startInfo)
   at GitVersion.Helpers.ProcessHelper.Start(ProcessStartInfo startInfo) in ProcessHelper.cs: line 26

So I created this branch to at least figure out that it was git it couldn't find, because as you can see from the stack trace, the Win32Exception that was originally thrown in isolate, is not very informative ("The system cannot find the file specified"; what file?).

Thus, this branch and PR does not fix the underlying problem, but at least makes the symptom of the problem discoverable. I hope we can discuss in this PR:

1. Why the git executable can't be found
2. Why it is needed
3. Why CONTRIBUTING.md doesn't mention this
4. What the recommended steps to fix the problem are
5. What I can do to help

😃

[JakeGinnivan]

Thanks for the PR. It is used for the git log command to give a dump of the git graph. We could make the DumpGraph() method catch this exception and the write out a warning instead of the graph.

The reason for it is that when the test fails, having the commit graph logged out in the test output is really handy. And afaik libgit2sharp can't format logs like the git log command. (@nulltoken, can we replace https://github.com/ParticularLabs/GitVersion/blob/master/GitVersionCore.Tests/Helpers/GitTestExtensions.cs#L23 with libgit2sharp calls?)

We should add a note to contributing.

[asbjornu]

@JakeGinnivan I see. I've added a try/catch to DumpGraph() and now all tests are green. If this is an acceptable solution, there shouldn't be any need to add a note to CONTRIBUTING.md. I do agree that using libgit2sharp would be a better solution than a (possibly unavailable) command line git.exe, though, given that (as you write) libgit2sharp gives us the log adequately formatted.

If this PR looks good to you, it would be awesome to have it merged so I can rebase off of it to continue the profiling and testing of #456! 😄

[JakeGinnivan]

All merged (including #456). Master should be in a good state now, if you could give it a run that would be awesome