internal/coverage/pods: sort counter files first by origin, then name

This patch fixes a problem with the way pods (clumps of related
coverage meta+counter data files) are collected, which was causing
problems for "go tool covdata subtract".

A subtract operation such as "go tool covdata subtract -i=dir1,dir2
-o=out" works by loading in all the counter data files from "dir1"
before any of the data files from "dir2" are loaded. The sorting
function in the pods code was sorting counter files for a given pod
based purely on name, which meant that differences in process ID
assignment could result in some files from "dir2" being presented
before "dir1". The fix is to change the sorting compare function to
prefer origin directory over filename.

Fixes #60526.

Change-Id: I2226ea675fc99666a9a28e6550d823bcdf2d6977
Reviewed-on: https://go-review.googlesource.com/c/go/+/499317
Run-TryBot: Than McIntosh <thanm@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Michael Pratt <mpratt@google.com>

src/internal/coverage/pods/pods.go

@@ -166,6 +166,9 @@ func collectPodsImpl(files []string, dirIndices []int, warn bool) []Pod {
 	pods := make([]Pod, 0, len(mm))
 	for _, p := range mm {
 		sort.Slice(p.elements, func(i, j int) bool {
+			if p.elements[i].origin != p.elements[j].origin {
+				return p.elements[i].origin < p.elements[j].origin
+			}
 			return p.elements[i].file < p.elements[j].file
 		})
 		pod := Pod{