refactor: collapse statements in single assignments

Problem:

Variables are often assigned multiple places in common patterns.

Solution:

Replace these common patterns with different patterns that reduce the
number of assignments.

Use `MAX` and `MIN`:
```c
if (x < y) {
  x = y;
}

// -->

x = MAX(x, y);
```

```c
if (x > y) {
  x = y;
}

// -->

x = MIN(x, y);
```

Use ternary:
```c
int a;
if (cond) {
  a = b;
} els {
  a = c;
}

// -->

int a = cond ? b : c;

```

src/nvim/usercmd.c

@@ -805,9 +805,7 @@ invalid_count:
         }
       }
 
-      if (*def < 0) {
-        *def = 0;
-      }
+      *def = MAX(*def, 0);
     } else if (STRNICMP(attr, "complete", attrlen) == 0) {
       if (val == NULL) {
         semsg(_(e_argument_required_for_str), "-complete");