[FileCheck] In input dump, elide only if ellipsis is shorter

For example, given `-dump-input-context=3 -vv`, the following now
shows more leading context for the error than requested because a
leading ellipsis would occupy the same number of lines as it would
elide:

```
<<<<<<
         1: foo6
         2: foo5
         3: foo4
         4: foo3
         5: foo2
         6: foo1
         7: hello world
check:1     ^~~~~
check:2           X~~~~ error: no match found
         8: foo1
check:2     ~~~~
         9: foo2
check:2     ~~~~
        10: foo3
check:2     ~~~~
         .
         .
         .
>>>>>>
```

Reviewed By: mehdi_amini

Differential Revision: https://reviews.llvm.org/D83526