Add fish_job_summary, called whenever a job ends, stops, or is signalled

This allows users to customise the behaviour of the shell by redefining the function. This is similar to how fish_title or fish_greeting behave, where the default implementation can be easily overridden.

The function receives as arguments the job id, command line, signal name and signal description.

tests/job_summary.expect

@@ -0,0 +1,39 @@
+# vim: set filetype=expect:
+#
+# Test job summary for interactive shells.
+
+set pid [spawn $fish]
+expect_prompt
+
+send_line "function fish_job_summary; string join ':' \$argv; end"
+expect_prompt
+
+# fish_job_summary is called when background job ends.
+send_line "sleep 0.5 &"
+sleep 0.050
+expect_prompt
+sleep 0.550
+expect -re "\[0-9]+:sleep 0.5 &:ENDED"
+send_line ""
+expect_prompt
+
+# fish_job_summary is called when background job is signalled.
+# cmd_line correctly prints only the actually backgrounded job.
+send_line "false; sleep 10 &; true"
+sleep 0.100
+expect_prompt
+exec -- pkill -TERM sleep -P $pid
+sleep 0.100
+expect -re "\[0-9]+:sleep 10 &:SIGTERM:Polite quit request"
+send_line ""
+expect_prompt
+
+# fish_job_summary is called when foreground job is signalled.
+# cmd_line contains the entire pipeline. proc_id and proc_name are set in a pipeline.
+send_line "true | sleep 6"
+sleep 0.100
+exec -- pkill -KILL sleep -P $pid
+sleep 0.100
+expect -re "\[0-9]+:true | sleep 6:SIGKILL:Forced quit:\[0-9]+:sleep"
+send_line ""
+expect_prompt