compare: add --decorate

When a project is not on the `manifest-rev` it will often be on a branch
or tag. Use the `%d` format to make `west compare` show that
information.

As discussed in #643 and before, git status cannot be fully trusted to
display branch information.

Sample new output:
```
west compare
=== zephyr (zephyr):
--- manifest-rev: e40859f78712 (some_tag) Revert "dma: dw: Do ...
            HEAD: e803b77463b4 (zephyrproject/main) samples: net: ...
--- status:
    HEAD detached at zephyrproject/main
    nothing to commit, working tree clean
```

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

src/west/app/project.py

@@ -736,10 +736,16 @@ def print_rev_info(self, project):
             return
 
         def rev_info(rev):
-            return project.git(
-                ['log', '-1', '--color=never', '--pretty=%h %s', rev],
+            title = project.git(
+                ['log', '-1', '--color=never', '--pretty=%h%d %s',
+                 '--decorate-refs-exclude=refs/heads/manifest-rev',
+                 rev],
                 capture_stdout=True,
                 capture_stderr=True).stdout.decode().rstrip()
+            # "HEAD" is special; '--decorate-refs-exclude=HEAD' doesn't work.
+            # Fortunately it's always first.
+            return title.replace('(HEAD) ', '').replace('(HEAD, ', '(')
+
         head_info = rev_info('HEAD')
         # If manifest-rev is missing, we already failed earlier.
         manifest_rev_info = rev_info('manifest-rev')