If the command git is unvailable, username/password dialog throws an exception

[yann-gael]

When doing a commit, if the command git is unavailable, a dialog appears asking for a GitHub username/password. Filling-in and okaying this dialog will lead to an exception.

Step to reproduce:
(From memory because I don't have Pharo in front of me right now.)

• Make sure that the git command is unavailable to Pharo (e.g., from a terminal/console, git must not be found.)
• Select a class
• Right mouse button -> Commit
• Provide a message and click "Commit"
• A dialog appears asking for a username/password
• Enter anything here and "Okay"
• An exception is thrown

What was expected:
I think that Iceberg should check for git being available or not (maybe by just running git -v?) and should warn the user that she cannot commit anything (or do anything else) until git is available.

Stack and screenshot in attachment.
Stack.txt