Fix Clippy warning

[bjacotg]

The warning was:

warning: variant name ends with the enum's name
   --> googletest/src/matcher_support/summarize_diff.rs:232:5
    |
232 |     CommonLineBuffer(Vec<&'a str>),
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    |
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#enum_variant_names
    = note: `#[warn(clippy::enum_variant_names)]` on by default