*sh: add `ts` script
*sh: add `ts` script

Sometimes, I want to override the date on a commit. You can do this with
the `--date` option, but that only sets the author date. To set the
committer date you have to use the `GIT_COMMITTER_DATE` environment
variable.

I've found that, while `--date` and other places accept date strings
like "2.hours.ago", `GIT_COMMITTER_DATE` needs to follow a stricter
format as documented in the `git-commit` man page. Additionally, I've
found that nonsensical combinations like `GIT_COMMITTER_DATE` in the
past and `GIT_AUTHOR_DATE` in the present get silently ignored (and
`GIT_AUTHOR_DATE` is used for both). So, you have to pass both it seems.

In any case, I mad a quick and dirty shell script to make it easier to
do this. Fox example:

```
$ TS=$(ts -30m)
$ GIT_COMMITTER_DATE=$TS GIT_AUTHOR_DATE=$TS git commit
```

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