branch download completely fails when connection is interrupted

[veddox]

When branching from an external repository (e.g. branching Bazaar itself from Launchpad), all downloaded data is stored in memory until the download is completed. However, if the connection is lost (a frequent occurrence in some places), nothing gets written to disk. Thus, the user has to reattempt the entire download.

This brings two disadvantages:
a) Bazaar's memory usage sky-rockets - in the example given, it was sitting at ~280MB
b) In areas where Internet access is costly/unreliable, branching a large project may prove very difficult.

Suggested fix:
If possible, downloaded data should be written to disk immediately. This frees up memory and should make interrupted downloads feasible (as with a download manager).