ignore: fix dangling backslash error for escaped trailing space with extra spaces

[HarnageaGabriel]

Fixes #3477.

Problem

Git's gitignore trailing-whitespace handling only strips unescaped trailing spaces. An escaped trailing space (\ at the end of a pattern) is preserved even if followed by additional plain trailing spaces, e.g. a line consisting of foo + backslash + three spaces still keeps one escaped space after trimming (matches git check-ignore's behavior of foo\ -> matches foo ).

ripgrep's GitignoreBuilder::add_line only special-cased a line ending in exactly one \ (backslash immediately followed by a single space):

if !line.ends_with("\ ") {
    line = line.trim_right();
}

When there are extra unescaped spaces after the escaped one, the line no longer ends with \ (it ends with plain spaces), so it falls through to an unconditional trim_right(), which strips the escaped space too and leaves a dangling backslash — causing a parse error instead of ripgrep silently ignoring the pattern like git does.

Repro (from the issue)

$ printf 'foo\   \n' > .gitignore   # foo, backslash, three spaces
$ rg hi
rg: ./.gitignore: line 1: error parsing glob 'foo\': dangling '\'

Real git accepts this and ignores a file named foo (with trailing space).

Fix

Added trim_trailing_unescaped_spaces, a small byte-safe port of git's own trim_trailing_spaces() (dir.c): scan the line, treat \ + next byte as a consumed escaped pair (resetting any pending trailing-space run), track the start of a run of plain trailing spaces, and truncate at that point at the end of the scan (preserving a dangling backslash — with nothing to escape — as-is, matching git's early-return behavior for that case). This replaces the old ends_with("\ ") special case.

Verified against the reporter's exact repro plus manual testing with a real rg build: foo is now ignored while foo (no trailing space) and foo (extra unescaped trailing spaces) are not, matching git check-ignore output.

Testing

• Added three regression tests in crates/ignore/src/gitignore.rs: the reporter's exact multi-space repro, a plain-trailing-space-only regression check, and the pre-existing single escaped-space case.
• cargo test -p ignore: 191 passed.
• cargo test --workspace: all passing except two pre-existing, unrelated Windows-only symlink-privilege test failures (misc::symlink_nofollow, regression::r1389_bad_symlinks_no_biscuit) caused by the local environment lacking the symlink-creation privilege — unrelated to this change.
• Manual end-to-end repro with a built rg binary confirms git-matching behavior.

Note: a previous attempt at this fix (#3485) was opened and later closed by its own author; this is a fresh implementation using git's actual trimming algorithm as the reference rather than a heuristic patch.