Always add parent directories of files to snapshots.

[gilbsgilbs]

Fixes #1163

Description

Always add parent directories of files to snapshots.

During a snapshot, when a file changed and not its parent directories,
the parent directories weren't added to the layer. This is inconsistent
with Docker's behavior which always add parent directories to the layer.
In some edge-cases, it could lead to problems with docker considering
that parent directories where owned by root in forthcoming layers
although they shouldn't (see #1163).

Also, Docker seems to be POSIX compliant regarding the name of
directories in the archive, which always have a slash appended. This
commit also fixes this.

Fixes #1163

Submitter Checklist

These are the criteria that every PR should meet, please check them off as you
review them:

• Includes unit tests
  note: I just updated existing tests which I think is enough given how they all started to fail. I you don't think so, please tell me and I'll write a proper unit test.
• Adds integration tests if needed.

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Reviewer Notes

• The code flow looks good.
• Unit tests and or integration tests added.

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[gilbsgilbs]

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[gilbsgilbs]

Had to fix a merge conflict and reverse the order of ParentDirectories function as #1155 refactoring reversed it (for no apparent reason). Should be good now.

[azman0101]

Any news on this PR @tejal29 @cvgw ?

[tejal29]

Sorry for the delay @gilbsgilbs
LGTM!
I will do some more detailed testing with cache turned on and get this in by EOD tomorrow