Sometimes addtional repos are required to get necessary packages for
composing OSTree repository. For example, RHEL doesn't have an 'Everyting'
variant, so composing OSTree repository from any of the RHEL variants
won't work, addtional source repos need to be enabled to achieve that.
The new option "extra_source_repos" enable the ability of allowing extra
source repos.
And a new option 'keep_original_sources' is introduced to keep the
original repos found in tree config file, if this is enabled, Pungi
will not remove the existing source repos from the tree config file,
just add new repos of "source_repo_from" + "extra_source_repos" to
the existing repos.
Signed-off-by: Qixiang Wan <qwan@redhat.com>
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