Issue title: x/review/git-codereview: support mailing multiple revisions with a single CL
 Issue body: ```bash
$ git mail
git-codereview: cannot mail: multiple changes pending; must specify commit on command line:
	cacb069 git-codereview: pass args to installHook
	e3eeef8 git-codereview: use codereview.cfg from working tree
	1ac8f8d git-codereview: fix TestPendingNoneBranch
	1c26cef git-codereview: disable failing tests
```

I will now proceed to run `git mail` individually on every single commit, in order, manually copying and pasting in the SHA1s. This happens all the time, at least for me. I'd like to be able to tell `git mail` to mail all of them, re-using the other flags (reviewer, trybot) for every commit:

Maybe:

```bash
$ git mail -r adg -trybot --all
```

@adg, opinions?

 Comments: 
 Comment 0: Never mind. I'm an idiot. Can't believe it has taken me how long?? to figure out that I can just git mail the topmost commit. Sigh.
 Comment 1: You're not an idiot @josharian.
The error message is not helpful. This is a valid usability bug.
Adding something like "(use latest commit to submit them all)" would be simple and helpful.
 Comment 2: CL welcome!
 Comment 3: Keep this open as it's a valid issue.
 Comment 4: I also completely failed to understand that this is how it's supposed to be used.

Here's a question - why shouldn't `git-codereview mail` (with no arguments) just mean `git-codereview HEAD`? It seems to me like that's what people would understand it to mean. For example, `git push` with no arguments also pushes the head of the current branch.
 Comment 5: That sounds good to me. Alternatively I’d be happy enough with a better error message and accepting any git ref (eg “head”).
 Comment 6: Wait a minute, but `mail HEAD` works for me:
```
~/golang (ssa_cfg $%) % git mail HEAD
remote: Processing changes: refs: 1, updated: 2, done
remote:
remote: SUCCESS
remote:
remote: Updated Changes:
remote:   https://go-review.googlesource.com/c/go/+/142517 cmd/compile: add control flow graphs to ssa.html
remote:   https://go-review.googlesource.com/c/go/+/144557 cmd/compile: make ssa blocks collapsable in ssa.html
remote:
Updated tag 'ssa_cfg.mailed' (was 7794b8dadd)
```
Do we want to make `mail` command to mail the `HEAD` by default in case of the chain of changes?
 Comment 7: Change https://golang.org/cl/148137 mentions this issue: `git-codereview: add a hint about mailing the whole chain of changes`
 Comment 8: This was fixed by the CL.
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