Resolve "Some log lines appear to be duplicates"

Closes #507

See merge request !573

Since we have now backported this to `bst-1.2`, the APIs have
been introduced in 1.2 and not 1.4

These features will be released in 1.1.5 since they have been
backported, lets reflect this in the NEWS file in master.

Also this adds the backported `remote` plugin to the NEWS

Use os.rename() to rename the cloned temporary repository into
place in the source cache, and issue a STATUS message when discarding
a duplicate clone, in the case where the same repository is cloned
twice in parallel.

The problem with using shutil.move() is that it will create the source
directory in a subdirectory of the destination when the destination
exists, so it's behavior depends on whether the destination exists.

This shutil.move() behavior has so far hidden the race condition
where a duplicate repo is created in a subdirectory, as you need
to have three concurrent downloads of the same repo in order to
trigger the error.

This fixes issue #503

git.py: Handle concurrent download completions properly

Closes #503

See merge request !583

Phil Dawson authored Jul 10, 2018 and Phil Dawson committed Jul 30, 2018

This is part of the work towards #437.

Phil Dawson authored Jul 11, 2018 and Phil Dawson committed Jul 30, 2018

This is part of the work towards #437.

Phil Dawson authored Jul 17, 2018 and Phil Dawson committed Jul 30, 2018

This is part of the work towards #437.

Phil Dawson authored Jul 19, 2018 and Phil Dawson committed Jul 30, 2018

This is part of the work towards issue #437.

Phil/437 junction tutorial

See merge request !550

Source plugin implementations can now specify that they need access to
previously staged sources by specifying
`requires_previous_sources_track` and/or
`requires_previous_sources_fetch`, corresponding to access at `track`
and `fetch` times respectively.

Fixes #381.

Replaces !505. For relevant discussion, see this discussion:
!505 (comment 83780747)