[PATCH 0 of 3 V4] extensions: isolate each local repositories handled in a same process at a time

[FUJIWARA Katsunori]

This patch series fixes some problem about isolation between each
local repositories handled in a same process at a time.

Difference from V3:

    - drop the patch arond subrepo, which is already imported as the
      one of the Simon Heimberg's series to fix subrepo configuration
      problem

    - adjust patches for recent default branch