Update the value of the `diffEnd` when click `Show More` btn in the DiffFileTree...

Update the value of the `diffEnd` when click `Show More` btn in the DiffFileTree (#24069)

In the component `DiffFileTree`,if don't update the value of the
`diffEnd` in the callback of ajax request, click `Show More` btn will
always return the same response, duplicate files are appended to the
file list.

Before:

https://user-images.githubusercontent.com/33891828/231371188-82d169af-10bb-47e2-8aca-83ced2597f2d.mov

After:

https://user-images.githubusercontent.com/33891828/231369805-39a5a4d0-662c-4f08-bc5a-7d31e8782453.mov

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Co-authored-by: Giteabot <teabot@gitea.io>
Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>