fix: throw FormatException on malformed commit/tree output and add fuzz workflow

[kevmoo]

Rationale

Coverage-guided fuzzing of package:git uncovered 4 cases where TreeEntry.fromLsTree, Commit.parse, and Commit.parseRawRevList threw StateError, null-check TypeError, ArgumentError, or AssertionError instead of FormatException when given malformed git ls-tree or git rev-list --header output.

Summary of Changes

• Replace _lsTreeRegEx.allMatches(value).single in TreeEntry.fromLsTree (lib/src/tree_entry.dart) with _lsTreeRegEx.firstMatch(value) and throw FormatException when no match is found.
• Validate required single headers (tree, author, committer, and commit in rev-list mode), SHA-1 header format, and trailing commit message newline in Commit._parse (lib/src/commit.dart) via scanner.error (FormatException) instead of throwing null-check TypeError, StateError, ArgumentError, or AssertionError.
• Pass --no-sign when creating a lightweight tag in test/tag_test.dart so global tag.gpgSign=true configurations do not prompt for an annotated tag message.
• Add unit test regressions in test/parse_test.dart, a coverage-guided fuzz target in test/fuzz/git_parser_fuzz.dart, and .github/workflows/fuzz.yaml (instrument-packages: string_scanner,source_span).

Verification

• Executed dart test (100% pass).
• Executed 5s CGF fuzz run with --instrument-packages=string_scanner,source_span (149,978 executions, ~25,000 exec/s, 0 crashes, 71.0% AST coverage on commit.dart and 60.0% on tree_entry.dart).
• Executed kscripts pr-check with 0 issues.

[github-actions]

📊 Cognitive Complexity Analysis

Net Delta: +8 | Added: 1 | Increased: 2 | Improved: 0 | Removed: 0 | Violations: 0

Status	Declaration	Location	Delta	Score
🟡	Commit._parse	lib/src/commit.dart:L57-121	+2	11 -> 13
🔵	Commit._singleHeader	lib/src/commit.dart:L123-141	+5	new -> 5
🟡	TreeEntry.fromLsTree	lib/src/tree_entry.dart:L35-42	+1	0 -> 1