Fixes#124.
Since #78 the shell reports `IRIS_CWD` on `chpwd`, but the wrapper only
passes it to
`spec.SetCWD` for suggestions. Its own working directory never moves, so
tools that
read the pane's foreground process to locate the shell resolve the wrong
path after a
`cd`. I hit this with herdr, which stopped detecting the git repository
of a pane once
IRIS was in it. tmux uses the same lookup for `pane_current_path`.
The absolute path check matches what `spec.SetCWD` already does, so both
treat a
malformed value the same way. The `os.Chdir` error is dropped because
the path comes
from the shell integration, the user cannot act on the failure, and
keeping the last
known directory is better than stopping the loop.
`go test ./...` passes. I tested this end to end on macOS only: the
wrapper followed
the `cd`, and herdr picked the repository back up.
One limitation: the outer launcher process keeps its start directory,
only the wrapper
running `runWrapper` follows. That was enough for herdr, which reads the
wrapper.
