git add -p: use non-zero exit code when the diff generation failed

The first thing `git add -p` does is to generate a diff. If this diff cannot be generated, `git add -p` should not continue as if nothing happened, but instead fail. What we *actually* do here is much broader: we now verify for *every* `run_cmd_pipe()` call that the spawned process actually succeeded. Note that we have to change two callers in this patch, as we need to store the spawned process' output in a local variable, which means that the callers can no longer decide whether to interpret the `return <$fh>` in array or in scalar context. This bug was noticed while writing a test case for the diff.algorithm feature, and we let that test case double as a regression test for this fixed bug, too. Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de> Signed-off-by: Junio C Hamano <gitster@pobox.com>