bundle: add an experimental knob to include obsmarkers in bundle
The "hg bundle" command is a good place to test if the inclusion of obsmarkers
within a bundle is working well (part exists, content is correct etc). So we
add a way to have them included.
Ideally, this would be controlled by a change around bundlespec (bundlespec
"v3" + arguments). However, my main goal is to have obsmarkers included in
bundle created by the 'hg strip' command, not the 'hg bundle' so for now I'm
avoiding the detour through bundlespec rework territory.
Better debug output for obsmarkers in 'debugbundle' will be added in later
changesets. The 'test-obsolete-bundle-strip.t' test will also get updated in a
later changeset to keep the current changeset smaller.