The git log features have a way to remap author and committer names and email address into canonical names, via the creation of a .mailmap file. For more details, see the help for git shortlog.
So, you know you need one of these – how do you create it? Well, use the output from git shortlog to do this, and feed it into perl or sed to remove the counts at the beginning of each line. For example
git shortlog -se | perl -ple "s/^\s*\d+\t//" > .mailmap.txt
(on Windows, you’ll need to use double-quotes “” instead of single-quotes ” to contain the string).
This leaves you with the start of the .mailmap file. Now, you just edit it; pick the canonical name you want, and create any mappings needed to turn the non-canonical entries into your canonical ones.
Note that this can be very tedious for large repos with hundreds or even thousands of contributors (the git.git repo has just over 1200 contributors as of 2014). You might want to focus on the oddballs. In that case, add -n to the shortlog invocation – this will sort by occurrence instead of alphabetically. This way, you can go from the bottom of the list until it’s “good enough”. Once you’ve edited it, you probably want to sort it alphabetically, to make future life easier.
You can commit the .mailmap file to your repository so that others can benefit from what you’ve done, or you can leave it for just your use.
As for updating it in the future, I’d suggest diffing the output of shortlog “now” and back at the point in time where you last updated the .mailmap, to see if you have any new authors that need remapping.
Note that mailmap files aren’t automatically used by anything other than shortlog. For example, for git log, you either need to put it in the config, or add it to the git log command line
git log --use-mailmap [more options]