push: make pushkey part advisory
The current behavior (with bundle1) is to let the rest of the push succeed if
the pushkey call (phases, bookmarks) failed (this comes from the fact that each
item is sent in its own command).
We kept this behavior with bundle2, which is highly debatable, but let us keep
thing as they are now as a start. We are about to enforce 'mandatory' pushkey
part as 'mandatory' successful, so we need to marks parts as advisory to
preserve the current (debatable) behavior.
bundle2: pull bookmark the old way if no bundle2 listkeys support (
issue4701)
All known server implementations have listkeys support with bundle2, but people
in the process of implementing new servers may not. Let's be nice with them.
tests: simplify printenv calls
Make printenv executable so that we don't need python, TESTDIR, or
quoting.
tests: simplify calls to dummyssh
dummyssh is marked executable and is in the path, no need for python,
TESTDIR, or quotes.