don't try to make push on read-only url

[Eric Seigne]

bzr push on http:// external branches make a warning message "bzr: ERROR: Cannot lock LockDir(http://....../.bzr/branch/lock): Transport operation not possible: http does not support mkdir()"

so it could be a bzr "bug" or a bzr-externals one ...

[Eugene Tarasenko (etarasenko)]

Is bzr+http access to repository always read only?

[Reagan Sanders (vexo)]

http access can be read-write via WebDAV.

I think this issue is actually something else entirely: when the root branch is pushed to a specified location, the external branch is also pushed, but it uses its default push location. The correct behavior would probably be to calculate the correct sub-directory of the push location specified for the parent branch based on the external branch's relative path.