Fix git grep -l replacing first hyphen with colon in filenames

[cjpeterein]

Summary

Fix for issue #1674: git grep -l output incorrectly replaces the first hyphen in filenames with a colon (e.g., my-file.rs becomes my:file.rs).

• Detect when git grep is running with filename-only output flags (-l, -L, --files-with-matches, --files-without-match)
• Skip grep line parsing entirely for these modes, allowing filenames to pass through unchanged

Problem

When running git grep -l "pattern" | delta, filenames containing hyphens are corrupted:

# Expected output
my-file.rs

# Actual output before fix
my:file.rs

Root cause: The WithoutSeparatorCharacters regex in parse_grep_line() excludes hyphens from the file path character class. For input my-file.rs (with no line number or code content), it matches:

• File path: my (stops at hyphen)
• Separator: - (the hyphen from the filename)
• Code: file.rs

When the separator is replaced with the default :, the filename becomes my:file.rs.

Solution

Detect filename-only output mode by checking for -l/-L/--files-with-matches/--files-without-match flags in the calling process command line. When detected, return None from parse_grep_line() so the line passes through unchanged.

Alternatives Considered

Option B: Improve regex to require content after separator

Modify the WithoutSeparatorCharacters regex to require actual code content after the separator, not just a file extension pattern.

Ruled out because:

• The regex is already complex and fragile
• The ignored test at line 909 (test_parse_grep_n_match_file_name_with_dashes_and_no_extension) demonstrates this is a known hard problem
• Risk of breaking other edge cases
• Doesn't address the semantic issue: -l output simply isn't grep output format

Option C: Validate parsed path exists on filesystem

After parsing, check if the extracted file path actually exists before accepting the parse result.

Ruled out because:

• Performance impact from filesystem calls on every line
• Doesn't work for grep output piped from remote sources or stdin
• Doesn't work for files that have been deleted since the grep ran
• Over-engineered solution for a simpler problem

Test Plan

• New test: test_parse_grep_filename_only_output_with_hyphen - verifies filenames with hyphens return None when -l, -L, --files-with-matches, or --files-without-match flags are present
• New test: test_parse_grep_regular_output_with_hyphen_still_works - verifies regular grep output with line numbers still parses correctly
• All existing grep tests pass
• Manual verification: git grep -l "pattern" | delta preserves hyphenated filenames

Fixes #1674

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