diff: pass --no-color so forced git colors do not empty the diff body - #96363

With color.ui=always or color.diff=always in a repository's or a person's git config, every git diff the mod runs answers with ANSI escapes. The header and the per-file counts stay right (they come from --shortstat and --numstat, which git never colors), but no line matches the hunk header pattern any more, so every file's body is empty and the pane shows No diff content under each changed file.

--no-color now rides the shared diff arguments (DIFF_LEADING_ARGS), which every git diff child of the mod spreads (the stats pass, the numstat listing, a file's hunks), so they all answer plain text whatever either setting says. The argv's doc says what the flag pins. The new test in tests/git/fetch-diff.test.ts scripts a colored answer for a body fetch without the flag and a plain one with it, and expects the file's hunk.

Checked:

• A real temporary repository with color.ui=always, then with color.diff=always, diff.colorMoved=zebra and a custom color.diff.meta: before, /diff from claude --plugin-dir mods/diff shows 2 files changed +2 -1, the two rows with their counts, and No diff content under each; after, each file's hunks. Without either setting the pane reads the same before and after.
• The new test fails with the argv line reverted (1 of 156) and passes with it.
• tsc -p mods/tsconfig.json, claude plugin validate mods/diff, git diff --check, and claude plugin test on all four mods (207 tests) under 2.1.277.

Patch by @7487, from #91870.