Add dependabot to .mailmap.

Before:
```
$ git shortlog -se | grep "depend"
    44	dependabot-preview[bot] <27856297+dependabot-preview[bot]@users.noreply.github.com>
     6	dependabot-preview[bot] <dependabot-preview[bot]@users.noreply.github.com>
     4	dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>
    74	dependabot[bot] <dependabot[bot]@users.noreply.github.com>
    93	dependabot[bot] <[email protected]>
```
After:
```
$ git shortlog -se | grep "depend"
   221  dependabot[bot] <dependabot[bot]@users.noreply.github.com>
```

Author: nnethercote

.mailmap

@@ -141,6 +141,10 @@ David Ross <[email protected]>
 David Wood <[email protected]> <[email protected]>
 Deadbeef <[email protected]>
 Deadbeef <[email protected]> <[email protected]>
+dependabot[bot] <dependabot[bot]@users.noreply.github.com> <27856297+dependabot-preview[bot]@users.noreply.github.com>
+dependabot[bot] <dependabot[bot]@users.noreply.github.com> <49699333+dependabot[bot]@users.noreply.github.com>
+dependabot[bot] <dependabot[bot]@users.noreply.github.com> <dependabot-preview[bot]@users.noreply.github.com>
+dependabot[bot] <dependabot[bot]@users.noreply.github.com> <[email protected]>
 Derek Chiang <[email protected]> Derek Chiang (Enchi Jiang) <[email protected]>
 DeveloperC <[email protected]>
 Devin Ragotzy <[email protected]>