Allow tags to be added using date of commit instead of current date

When adding a new tag Sourcetree uses the current date on that tag. The tag date is then used by the Github UI for ordering releases. If tags were not added chronologically, the releases are displayed out of sequence. 

This can be avoided by setting GIT_COMMITTER_DATE to the date of the commit before adding the tag. 

I suggest adding this as an option under the Advanced Options of the Tag dialog.