Test: check that INCR object sharing works as expected.

tests/unit/basic.tcl

@@ -150,6 +150,29 @@ start_server {tags {"basic"}} {
         r decrby novar 17179869185
     } {-1}
 
+    test {INCR uses shared objects in the 0-9999 range} {
+        r set foo -1
+        r incr foo
+        assert {[r object refcount foo] > 1}
+        r set foo 9998
+        r incr foo
+        assert {[r object refcount foo] > 1}
+        r incr foo
+        assert {[r object refcount foo] == 1}
+    }
+
+    test {INCR can modify objects in-place} {
+        r set foo 20000
+        r incr foo
+        assert {[r object refcount foo] == 1}
+        set old [lindex [split [r debug object foo]] 1]
+        r incr foo
+        set new [lindex [split [r debug object foo]] 1]
+        assert {[string range $old 0 2] eq "at:"}
+        assert {[string range $new 0 2] eq "at:"}
+        assert {$old eq $new}
+    }
+
     test {INCRBYFLOAT against non existing key} {
         r del novar
         list    [roundFloat [r incrbyfloat novar 1]] \