Fix error in IR stable names Git diff CI script (#7954)

#7644 added a script that gets run in CI to display a human-readable
diff if `source/slang/slang-ir-insts-stable-names.lua` needs to be
updated. However, the `git diff` commands in that script are wrong,
causing it to instead just display the man page for the `git diff`
command; [here's an
example](https://github.com/shader-slang/slang/actions/runs/16578560460/job/46888822691):

```
=== Updating stable names ===
Added 1 new instructions to source/slang/slang-ir-insts-stable-names.lua

=== Diff of changes made ===
usage: git diff --no-index [<options>] <path> <path>

Diff output format options
```

(followed by many lines)

This PR fixes the script. Here's an example showing it working correctly
after the fix:

-
https://github.com/shader-slang/slang/actions/runs/16578977233/job/46890240012
- https://github.com/shader-slang/slang/actions/runs/16578977233

Co-authored-by: Ellie Hermaszewska <ellieh@nvidia.com>

0 files changed, 0 insertions, 0 deletions