populate: exit with the status of the command that failed

The shell implementation runs under set -e, so it exits with the status
of the command that failed, e.g. 255 if ssh can't reach the target,
which wrappers retrying on connection failures rely on, or 128 if git
fetch fails.  The program does the same, instead of exiting with 1, and
with 128 plus the signal for a command a signal ended.

A phase's expect.status is the exit status it expects populate to fail
with; git-status checks git fetch's.

1 files changed, 5 insertions, 1 deletions

diff --git a/pkgs/populate/docs/design.md b/pkgs/populate/docs/design.md
index 6ad5eea..cc78a65 100644
--- a/pkgs/populate/docs/design.md
+++ b/pkgs/populate/docs/design.md
@@ -37,7 +37,11 @@ populate SPEC
 input.  Exit status:
 
 - 0: populate delivered all sources.
-- 1: a check of the spec failed, or delivering did.
+- 1: a check of the spec failed, or delivering did, with a message of its
+  own.
+- The exit status of the command that failed, if delivering failed with
+  that, like the shell implementation does: e.g. 255 if ssh can't reach
+  the target, or 128 if git fetch fails.
 - 2: the arguments are wrong, or the spec can't be read, isn't JSON, is of
   another version, or doesn't have the spec's shape.
 - 129, 130, 143: populate got SIGHUP, SIGINT, or SIGTERM, and stopped.