msubprojects: if fetching the remote fails, gracefully fallback to local copy

This command is useful to e.g. update a cloned subproject which does not
have its packagefiles merged, or which has updated packagefiles. This
does not strictly require internet, so if we can satisfy the ref, simply
log a warning and continue.

This enables a workflow where for network-isolated builds, the
subproject is cloned and moved into place and later on the build
orchestrator runs `meson subprojects update` without a network interface
to initialize the subproject.

mesonbuild/msubprojects.py

@@ -261,9 +261,14 @@ class Runner:
                 self.git_output(['fetch', '--refmap', heads_refmap, '--refmap', tags_refmap, 'origin', revision])
             except GitException as e:
                 self.log('  -> Could not fetch revision', mlog.bold(revision), 'in', mlog.bold(self.repo_dir))
-                self.log(mlog.red(e.output))
+                if quiet_git(['rev-parse', revision + '^{commit}'], self.repo_dir)[0]:
-                self.log(mlog.red(str(e)))
+                    self.log(mlog.yellow('WARNING:'), 'Proceeding with locally available copy')
-                return False
+                    # Trick git into setting FETCH_HEAD from the local revision.
+                    quiet_git(['fetch', '.', revision], self.repo_dir)
+                else:
+                    self.log(mlog.red(e.output))
+                    self.log(mlog.red(str(e)))
+                    return False
 
         if branch == '':
             # We are currently in detached mode