[PATCH 0 of 5 V2] pull, clone: add --subrepos flag

[Angel Ezquerra]

This is V2 of the previous "pull: add --subrepos flag" patch series.

This version also adds the --subrepos flag to clone. This made sense once it was added to pull. In addition this was necessary in order to be able to pull --subrepos recursively.

All tests (that were not skipped on my machine) pass, including the tests that I've added to test this new feature.