parsers: don't ask about the exec bit on platforms that don't have it
Otherwise the compiler gets quite sad.
Differential Revision: https://phab.mercurial-scm.org/D11711
tests: fix test-convert-git to work w/ "git pull" requiring strategy
A recent change to git (031e2f7ae195) made it an error to not specify a strategy
(`--rebase`, `--no-rebase`, `--ff-only`), instead of just the warning it was
previously. As far as I can tell, `--no-rebase` is the behavior we were getting
before, and the only one that makes the test work.
Differential Revision: https://phab.mercurial-scm.org/D11714