VCS: Fix display of diff/show commands in progress bar

In 6ae0e8d2c7 we added 2 arguments that
precede the diff sub-command (-c diff.color=false).

When display name is not set, ShellCommand uses the binary followed by the
first argument, so it now showed "Git -c" instead of "Git Diff".

Change-Id: Ie584837bd87fd529e0cd015025fc3bf1a69d085c
Reviewed-by: André Hartmann <aha_1980@gmx.de>
Reviewed-by: Xiaofeng Wang <wasphin@gmail.com>
Reviewed-by: Tobias Hunger <tobias.hunger@qt.io>

src/plugins/git/gitclient.cpp

@@ -291,6 +291,7 @@ GitDiffEditorController::GitDiffEditorController(IDocument *document, const QStr
 {
     connect(&m_decorator, &DescriptionWidgetDecorator::branchListRequested,
             this, &GitDiffEditorController::updateBranchList);
+    setDisplayName("Git Diff");
 }
 
 void GitDiffEditorController::updateBranchList()
@@ -514,7 +515,9 @@ public:
         GitDiffEditorController(document, dir),
         m_id(id),
         m_state(Idle)
-    { }
+    {
+        setDisplayName("Git Show");
+    }
 
     void reload() override;
     void processCommandOutput(const QString &output) override;