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

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>