t3904: fix cut-and-paste typo in test description
tests: make patch-mode test failure more verbose

Previously, a failing test of verify_state would simply exit
non-zero, causing the test to fail. This meant that it was
difficult to know which file had different contents, or what
the difference was. Instead, build it around test_cmp, which
generally shows a diff on failure.
stash apply -p WIP