Merge branch 'jk/diag-unexpected-remote-helper-death'

When a remote-helper dies before Git writes to it, SIGPIPE killed
Git silently.  We now explain the situation a bit better to the end
user in our error message.

* jk/diag-unexpected-remote-helper-death:
  print an error when remote helpers die during capabilities

diff --cc t/t5801-remote-helpers.sh
Simple merge

diff --cc transport-helper.c
Simple merge