Bug 7505

3 examples below:

Example 1: 
If you import a layer that has the same git url as an existing layer source layer, setting the revision to a specific commit sha, the build request fails with this error

command: git fetch --all && git checkout
"57c8e6bffbbef9b2331551093c3436303eb18386" && git rebase
"origin/57c8e6bffbbef9b2331551093c3436303eb18386"
From git://git.yoctoproject.org/meta-raspberrypi
* [new branch]      ag-dev     -> origin/ag-dev
* [new branch]      daisy      -> origin/daisy
* [new branch]      danny      -> origin/danny
* [new branch]      denzil     -> origin/denzil
* [new branch]      dizzy      -> origin/dizzy
* [new branch]      dora       -> origin/dora
* [new branch]      dylan      -> origin/dylan
* [new branch]      master     -> origin/master
Note: checking out '57c8e6bffbbef9b2331551093c3436303eb18386'.

You are in 'detached HEAD' state. You can look around, make experimental
changes and commit them, and you can discard any commits you make in this
state without impacting any branches by performing another checkout.

If you want to create a new branch to retain commits you create, you may
do so (now or later) by using -b with the checkout command again. Example:

  git checkout -b new_branch_name

HEAD is now at 57c8e6b... rpi-base.inc: Include the RPI2 DTB
fatal: Needed a single revision
invalid upstream origin/57c8e6bffbbef9b2331551093c3436303eb18386

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Example 2: 
If you import a layer with a git url that doesn't match any existing layers in Toaster, and set the revision to a specific commit sha, you get this error

command: git clone "https://github.com/agherzan/meta-raspberrypi"
--single-branch --branch "b896a7da70dd7a16ba7ffd664f7747cb37e1d142"
"/home/yocto/_meta-raspberrypi_b896a7da70dd7a16ba7ffd664f7747cb37e1d142.toaster_cloned"
Cloning into '/home/yocto/_meta-raspberrypi_b896a7da70dd7a16ba7ffd664f7747cb37e1d142.toaster_cloned'...
warning: Could not find remote branch
b896a7da70dd7a16ba7ffd664f7747cb37e1d142 to clone.
fatal: Remote branch b896a7da70dd7a16ba7ffd664f7747cb37e1d142 not
found in upstream origin
Unexpected end of command stream

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Example 3:
If you import a layer, and the checkout directory that Toaster creates happens to exist already but with a different origin, you get this error (if you delete the directory, the build works):

Existing git repository at
/home/yocto/_meta-raspberrypi_master.toaster_cloned, but with
different remotes ('origin git://git.yoctoproject.org/meta-raspberrypi
(fetch), origin git://git.yoctoproject.org/meta-raspberrypi (push), ',
expected 'https://github.com/agherzan/meta-raspberrypi'). Toaster will
not continue out of fear of damaging something.

*****************************

The fact that you can not build a layer when you set the revision to a certain commit sounds a bit worrying to me, but I could be totally wrong (maybe nobody would ever want to do something like that).