west diff: only print output for projects with nonempty diffs

The output is kind of cluttered when many projects are available.
Let's silence output for projects with no diff when not in verbose mode.

Signed-off-by: Martí Bolívar <marti.bolivar@nordicsemi.no>

src/west/app/project.py

@@ -556,16 +556,30 @@ class Diff(_ProjectCommand):
         self._setup_logging(args)
 
         failed = []
+        no_diff = 0
+        # We may need to force git to use colors if the user wants them,
+        # which it won't do ordinarily since stdout is not a terminal.
+        color = ['--color=always'] if log.use_color() else []
+
         for project in self._cloned_projects(args):
-            log.banner(f'diff for {project.name_and_path}:')
             # Use paths that are relative to the base directory to make it
             # easier to see where the changes are
-            try:
-                project.git(['diff', f'--src-prefix={project.path}/',
-                             f'--dst-prefix={project.path}/'])
-            except subprocess.CalledProcessError:
+            cp = project.git(['diff', f'--src-prefix={project.path}/',
+                              f'--dst-prefix={project.path}/',
+                              '--exit-code'] + color,
+                             capture_stdout=True, capture_stderr=True,
+                             check=False)
+            if cp.returncode == 0:
+                no_diff += 1
+            if cp.returncode == 1 or log.VERBOSE > log.VERBOSE_NONE:
+                log.banner(f'diff for {project.name_and_path}:')
+                log.inf(cp.stdout.decode('utf-8'))
+            elif cp.returncode:
                 failed.append(project)
-        self._handle_failed(args, failed)
+        if failed:
+            self._handle_failed(args, failed)
+        elif log.VERBOSE <= log.VERBOSE_NONE:
+            log.inf(f"Empty diff in {no_diff} projects.")
 
 class Status(_ProjectCommand):
     def __init__(self):