test-context: add a case demonstrating manifest caching problem This issue was discovered when testing absorb on Windows. What happens are: 1. ctx.p1().manifestctx gets cached. let's call ctx.p1().manifestctx._revlog() "mrevlog1" 2. repo.manifestlog gets invalidated. let's call repo.manifestlog._revlog "mrevlog2" 3. repo.commitctx(ctx) commitctx uses ctx.p1().manifestctx and writes to "mrevlog1" 4. repo[n].manifest() cannot find the manifest node in "mrevlog2" This patch adds a test case to reproduce the issue.