Fix #1798 - Disable UndoLastCommit when no commits

[Krzysztof-Cieslak]

This makes Undo Last Commit action in Git Integration if there are no commits (usually just after git init)

Unfortunately UndoLastCommit will also fail if there is only one commit (HEAD~ is not defined in such case). But it looks like that currently git integration doesn't provide information about HEAD parent (or commit list, or anything like that) so it would require more "deep" changes.

CC: @joaomoreno as assignee to #1798

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[coveralls]

Coverage remained the same at 61.185% when pulling 51e4376 on Krzysztof-Cieslak:fix_1798 into 8911a6a on Microsoft:master.

[bjtucker]

This makes Undo Last Commit action in Git Integration if there are no commits (usually just after git init)

Cool, thank you! This bug held a special place in my heart, since it lost me a little bit of data in my first hour or two of trying vscode.

Unfortunately UndoLastCommit will also fail if there is only one commit (HEAD~ is not defined in such case).

Does gitservice throw an error that you can trap if HEAD~ is not available? I see an IGitServiceError defined in https://github.com/Microsoft/vscode/blob/master/src/vs/workbench/parts/git/common/git.ts
but I can't dig in right now...

In such a case (deleting the first and only commit), I think the right thing to do would be to fall back to delete the branch. Perhaps with a confirmation prompt. Thoughts?

[joaomoreno]

Thanks!