app: project: fix git diff --exit-code logic

Fixes 2020 commit a53ec10 ("west diff: only print output for
projects with nonempty diffs")

Fixes #731

At least with git version 2.46.0, --exit-code is 0 when there has been a
merge conflict! git bug or feature? Either way this causes west diff to
wrongly assume there is no diff in that repo.

Two other issues:
1. if returncode is higher than 1 (= a git diff failure) AND -v is used,
   then failed.append(project) is never run.
2. stderr is never, ever printed

Try for instance `west diff --fubar` and `west -v --fubar` (which are
possible since commit 73aee32). One fails and the other does
not. Neither prints any useful error.

The verbosity level should never have any side-effect.

To fix:
- Assume there is a diff when --exit-code is zero AND stdout is not
  empty. Treat this the same as --exit-code=1
- Always `failed.append(project)` when --exit-code > 1
- Print stderr when there is a diff or in verbose mode.

This drops one `elif` which also simplifies the logic.

Signed-off-by: Marc Herbert <marc.herbert@intel.com>

src/west/app/project.py

@@ -89,6 +89,7 @@ def _handle_failed(self, args, failed):
         # and report errors if anything failed.
 
         if not failed:
+            self.dbg(f'{self.name} failed for zero project')
             return
         elif len(failed) < 20:
             s = 's:' if len(failed) > 1 else ''
@@ -804,13 +805,20 @@ def do_run(self, args, user_args):
                              extra_args=user_args,
                              capture_stdout=True, capture_stderr=True,
                              check=False)
-            if cp.returncode == 0:
+
+            # We cannot trust --exit-code alone, for instance merge
+            # conflicts return 0 with (at least) git version 2.46.0. See
+            # west issue #731
+            some_diff = cp.returncode == 1 or (cp.returncode == 0 and len(cp.stdout) > 0)
+            if not some_diff:
                 no_diff += 1
-            if cp.returncode == 1 or self.verbosity >= Verbosity.DBG:
+            if some_diff or self.verbosity >= Verbosity.DBG:
                 self.banner(f'diff for {project.name_and_path}:')
                 self.inf(cp.stdout.decode('utf-8'))
-            elif cp.returncode:
+                self.inf(cp.stderr.decode('utf-8'))
+            if cp.returncode > 1:
                 failed.append(project)
+
         if failed:
             self._handle_failed(args, failed)
         elif self.verbosity <= Verbosity.INF: