[poky] When SRCREV is set to AUTOREV, what triggers (re-)fetching?

[Evade Flow]

I have a recipe that sets `SRCREV = ${AUTOREV}`, organized almost exactly
like Rob Calhoun's template from this thread:

  - https://lists.yoctoproject.org/pipermail/yocto/2013-November/017042.html

It works as I expect, in that running `bitbake myprog` results in output
like:

```
Currently 1 running tasks (2441 of 2447):
0: myprog-1.0.0+gitAUTOINC+91b8e42702-r0 do_install (pid 17307)
```

However, if I push a change to the upstream so that the `HEAD` revision
moves, and immediately re-run `bitbake myprog`, I get:

```
NOTE: Preparing RunQueue
NOTE: Executing SetScene Tasks
NOTE: Executing RunQueue Tasks
NOTE: Tasks Summary: Attempted 2447 tasks of which 2447 didn't need to be
rerun and all succeeded.
NOTE: Writing buildhistory

Summary: There were 8 WARNING messages shown.
```

My hope was that bitbake would pull the new `HEAD` and rebuild, but it
seems 'stuck' on the revision it built first. Are my expectations for how
`AUTOREV` ought to work flawed? If so... what commands can I use to make
bitbake re-fetch the source?

Some details of my setup:

```
$ bitbake --version
BitBake Build Tool Core version 1.28.0

$ cat distro/poky/meta-yocto/conf/distro/poky.conf | grep DISTRO | head -4
DISTRO = "poky"
DISTRO_NAME = "Poky (Yocto Project Reference Distro)"
DISTRO_VERSION = "2.0"
DISTRO_CODENAME = "jethro"
```
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