compare: output improvements

A few concerns have been voiced about the west compare output:

   1. its signal:noise ratio is too low (too many lines per
      project without enough useful information per line)
   2. the end of the status output from one project is hard to
      separate from the status output for the next
   3. it isn't obvious enough whether HEAD == manifest-rev or not

To try to resolve 2. without making 1. any worse, explicitly print
HEAD and manifest-rev if they differ, without adding any extra lines,
by consolidating some of the output into the relevant banner line.

To solve 3., indent the status output to the same level as the
'status:' text in the banner.

Fixes: #650
Signed-off-by: Martí Bolívar <marti.bolivar@nordicsemi.no>

src/west/app/project.py

@@ -614,8 +614,9 @@ class Compare(_ProjectCommand):
             This command prints output for a project if (and only if)
             at least one of the following is true:
 
-            1. its checked out commit is different than the commit checked
-               out by the most recent successful "west update"
+            1. its checked out commit (HEAD) is different than the commit
+               checked out by the most recent successful "west update"
+               (which the manifest-rev branch will point to)
             2. its working tree is not clean (i.e. there are local uncommitted
                changes)
             3. it has a local checked out branch (unless the configuration
@@ -667,7 +668,7 @@ class Compare(_ProjectCommand):
                 self.config.getboolean('compare.ignore-branches', False)
 
         failed = []
-        printed_status = False
+        printed_output = False
         for project in self._cloned_projects(args, only_active=not args.all):
             if isinstance(project, ManifestProject):
                 # West doesn't track the relationship between the manifest
@@ -675,12 +676,8 @@ class Compare(_ProjectCommand):
                 # in printing output for comparisons that makes sense.
                 if self._has_nonempty_status(project):
                     try:
-                        self.banner(f'{project.name_and_path}:')
-                        self.small_banner('HEAD:')
-                        project.git('log --oneline -n 1 HEAD')
-                        self.small_banner('status:')
-                        project.git('status')
-                        printed_status = True
+                        self.compare(project)
+                        printed_output = True
                     except subprocess.CalledProcessError:
                         failed.append(project)
                 continue
@@ -714,22 +711,53 @@ class Compare(_ProjectCommand):
                         self._has_nonempty_status(project)):
                     continue
 
-                self.banner(f'{project.name_and_path}:')
-                # `git status` shows `manifest-rev` "sometimes", see
-                # #643. If manifest-rev is missing, then we already
-                # failed earlier anyway.
-                self.small_banner('manifest-rev:')
-                project.git('log --oneline -n 1 manifest-rev')
-                self.small_banner('status:')
-                project.git('status')
-                printed_status = True
+                self.compare(project)
+                printed_output = True
             except subprocess.CalledProcessError:
                 failed.append(project)
         self._handle_failed(args, failed)
 
-        if args.exit_code and printed_status:
+        if args.exit_code and printed_output:
             raise CommandError(1)
 
+    def compare(self, project):
+        self.banner(f'{project.name_and_path}:')
+        self.print_rev_info(project)
+        self.print_status(project)
+
+    def print_rev_info(self, project):
+        # For non-manifest repositories, print HEAD's and
+        # manifest-rev's SHAs and commit titles, but only if they are
+        # different.
+        #
+        # We force git not to print in color so west's colored
+        # banner() separators stand out more in the output.
+        if isinstance(project, ManifestProject):
+            return
+
+        def rev_info(rev):
+            return project.git(
+                ['log', '-1', '--color=never', '--pretty=%h %s', rev],
+                capture_stdout=True,
+                capture_stderr=True).stdout.decode().rstrip()
+        head_info = rev_info('HEAD')
+        # If manifest-rev is missing, we already failed earlier.
+        manifest_rev_info = rev_info('manifest-rev')
+        if head_info != manifest_rev_info:
+            self.small_banner(f'manifest-rev: {manifest_rev_info}')
+            self.inf(f'            HEAD: {head_info}')
+
+    def print_status(self, project):
+        # `git status` shows `manifest-rev` "sometimes", see #643.
+        if self.color_ui:
+            color = '-c status.color=always'
+        else:
+            color = ''
+        cp = project.git(f'{color} status', capture_stdout=True,
+                         capture_stderr=True)
+        self.small_banner('status:')
+        self.inf(textwrap.indent(cp.stdout.decode().rstrip(), ' ' * 4))
+
 class Diff(_ProjectCommand):
     def __init__(self):
         super().__init__(