convert: add config option to control storing original revision common.commit.__init__ sets saverev=True by default. The side effect of this is that the hg sink will always set the "convert_revision" extras key to the commit being converted. This patch adds a config option to disable this behavior. While most consumers will want "convert_revision" to be a) written b) with the exact Git commit that was converted, some have use cases that prefer otherwise. In my case, I am performing significant rewrites of a Git repository *before* it is fed into `hg convert`. I have to do this because `hg convert` does not easily support the kind of transform I desire, even with extensions. (For the curious, I am "linearizing" the history of a GitHub repo by removing merge commits which add little value to the final history. It isn't easy to do this during `hg convert` because of Mercurial's file copy/rename metadata requirements.) In my scenario, my pre-convert transform stores a "convert_revision" key in the Git commit object containing the original Git commit ID. I want this original Git commit ID carried forward to Mercurial. By disabling the setting of this extra during `hg convert` and copying the value from the Git commit object, I can have the final "convert_revision" extra key contain the original Git commit ID. An added test verifies this exact scenario. This feature could likely be implemented for other VCS sources. But until someone needs the feature, I'm inclined to hold off implementing.