RepoCommand: record manifest groups as submodule labels

RepoCommand: record manifest groups as submodule labels

Git core learned about attributes in pathspecs:

    pathspec: allow querying for attributes

    The pathspec mechanism is extended via the new
    ":(attr:eol=input)pattern/to/match" syntax to filter paths so that it
    requires paths to not just match the given pattern but also have the
    specified attrs attached for them to be chosen.

(177161a5f7, 2016-05-20)

We intend to use these pathspec attribute patterns for submodule
grouping, similar to the grouping in repo. So the RepoCommand which
translates repo manifest files into submodules should propagate this
information along. This requires writing information to the
.gitattributes file instead of the .gitmodules file. For now we just
overwrite any existing .gitattributes file and do not care about prior
attributes set. If this becomes an issue we need to figure out how to
correctly amend the grouping information to an existing .gitattributes
file.

Change-Id: I0f55b45786b6b8fc3d5be62d7f6aab9ac00ed60e
Signed-off-by: Stefan Beller <sbeller@google.com>