infinitepush: drop the `--create` flag to push command
The extension added a `--create` flag for creating a new bookmark on the
bundlestore. This patch changes the bahviour to create a bookmark if it does not
exists and removes the requirement of the `--create` and then drop the logic
around the `--create` flag.
Tests are changed to drop the usage of `--create` flag.
Differential Revision: https://phab.mercurial-scm.org/D2107
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