Show the run log's last lines under Details, live

The percentage says an update is alive. It does not say what it is alive
doing, and for the longest stretch of a run there was no way to find out:
an AUR package compiling for a quarter of an hour talks constantly, and
every word of it went into a file nobody was looking at.

So the run log's last five lines now travel with the progress entry and
sit under Details, refreshed every two seconds for as long as the update
lasts. Polled rather than followed, for the same reason the pacman
watcher polls: only the last few lines are ever displayed, so every line
in between would be work nobody sees, and re-reading the tail whole makes
truncation and rotation of the log a non-event.

The job model behind this interface carries exactly two description
fields - descriptionValue1 and 2, there is no third - so one stays with
the package being worked on and the other takes the tail. Newlines inside
a field do render, which is what lets five lines share one of them. They
travel tab separated because the protocol is one instruction per line,
and a tab inside a log line becomes a space first: either renders as
whitespace, but a line split in half renders as nonsense. Five lines
clipped to 120 columns also keeps an instruction well inside PIPE_BUF,
which is what makes the write atomic against the runner sending its own
progress down the same pipe.

src/cachy-auto-update-progress

@@ -8,6 +8,7 @@
 #
 #   info<TAB>text          headline, already translated by the caller
 #   detail<TAB>name<TAB>value  a labelled line under "Details"
+#   log<TAB>name<TAB>line<TAB>line...  the run log's last lines, one per field
 #   total<TAB>n            how many items this step has
 #   done<TAB>n             how many of them are finished
 #   percent<TAB>n          overall progress, 0-100
@@ -113,6 +114,16 @@ def main():
             elif cmd == "detail" and len(fields) > 2:
                 job.call("setDescriptionField",
                          GLib.Variant("(uss)", (0, arg, fields[2])))
+            elif cmd == "log" and len(fields) > 2:
+                # Field 1, and there is no field 2: the job model behind this
+                # interface carries exactly two, and anything further is
+                # discarded without complaint at the other end.
+                #
+                # The tail arrives one line per field, because the protocol
+                # itself is one instruction per line. Joined back up with the
+                # newlines the job view does render.
+                job.call("setDescriptionField",
+                         GLib.Variant("(uss)", (1, arg, "\n".join(fields[2:]))))
             elif cmd == "total":
                 job.call("setTotalAmount", GLib.Variant("(ts)", (int(arg), unit)))
             elif cmd == "done":