Handling of improperly-configured custom diff drivers is confusing

[Gesh <gesh@gesh.uni.cx>]

Hi,
After investigating what I thought was a git bug, I ultimately found it was due
to my misconfiguring the diff.pdf driver. In particular, I had (years ago now)
misread either the git docs or the pdftotext manual, because I had naively
configured
diff.pdf.textconv = pdftotext
Unfortunately, pdftotext's default behaviour is to write the text to a new file,
not stdout -- the correct configuration is
diff.pdf.textconv = sh -c 'pdftotext "$0" -'
All this is well and good, and thankfully this episode has left me with a better
configuration. Although, it would perhaps be nice to have git warn on a textconv
configuration that was constantly emitting an empty file for existent files.

Moreover, my investigation yielded some surprising (and underdocumented?)
information:
- It appears that triggering a textconv driver with cachetextconv set generates
  a commit. I caught this because I configure the user information per-repo, so
  diff was complaining the user wasn't configured
- Custom diff drivers are run in a temp directory subsequently rmdir'd by git.
  In particular, they _can_ write to disk, but must cleanup after themselves.
  Caught this because my configuration *was* leaving files around -- but git
  only complained it couldn't rmdir.
- git diff and git log crash if in the course of printing a custom diff driver
  crashes. In particular, they cut off printing. This was useful, because it
  meant I actually bumped into this bug, instead of it slipping my notice as I
  jumped in the output to the tag I was interested in.
- git log --simplify-by-decoration seems to generate some sort of squash commit
  of all the commits between two nodes it prints, but display them under the
  existing SHA. I think this was the first time I used simplify-by-decoration
  this thoroughly, so I hadn't noticed this before.

Not sure how much of this is actionable -- it ultimately ended up being my
fault, but I'd've appreciated clearer diagnostic messages and clarifications in
the docs.

Either way, hope this is useful to someone.
Gesh