Pass --no-ext-diff to internal git diff in standalone mode

[xfocus3]

Description

Closes #2046.

When delta is invoked as delta file1 file2, it builds and runs git diff --no-index --color ... and then colorizes git's diff output. That invocation reads the user's git config, so a configured diff.external (e.g. external = difft) takes over and delta ends up paging the external differ's output instead of producing its own — even when you're not inside a git repository.

Fix

Pass --no-ext-diff to the internal git diff invocation. This is git's canonical flag to ignore any configured diff.external for that command, so delta always receives git's own diff to format. The plain diff fallback path (used on git < 2.42 with process substitution) is unaffected since it doesn't read git config.

Verification

With diff.external configured to a marker script, before this change delta emitted the external differ's output; after it, delta produces its own diff and the external command is not invoked:

$ git config --global diff.external /tmp/fakediff.sh
$ delta a.txt b.txt
# before: output of /tmp/fakediff.sh
# after:  delta's own colorized diff (fakediff.sh not called)

cargo test (155 passed), cargo clippy, and cargo fmt --check all pass.