Make case-insensitive completion matches less important

fish --version: 2.7.1

## Scenes

#### scene 1

```bash
git status

modified: foo/Bar

git add B
# we got
git add foo/Bar

```
That's great.

#### scene 2

```bash

git status

modified: foo/Bar
modified: baz/Qux

git add B
# we got
git add baz/Qux

```
That's terrible.

#### What's we want

```bash

git status

modified: foo/Bar
modified: baz/Qux

git add B
# we got
git add baz/Qux

git add b
# we got
git add baz/Qux

```

With case-insensitive completion, we have worries more then benefits. So can we disable it with some configurations?