Don't erase commit message when commit fails

[fcollonval]

When using git hook (for example such as black), it may happen that the commit is rejected. Unfortunately in case of error the commit message is erased. It would be nice to keep it as the user will want to carry small correction (like add the black format correction) before retrying to commit.

[krassowski]

The relevant code is in:

jupyterlab-git/src/components/CommitBox.tsx

Lines 147 to 153 in f60ed32

	private _onCommitSubmit = (): void => {
	const msg = this.state.summary + '\n\n' + this.state.description + '\n';
	this.props.onCommit(msg);
	// NOTE: we assume here that committing changes always works and we can safely clear component state
	this._reset();
	};

The _reset() should only be called upon successful completion of the promise returned by onCommit(msg).