Refactor: Harden parseLsFiles against ReDoS

[aka76bm]

The previous implementation of parseLsFiles used a regular expression
to parse the output of git ls-files. While the regex was already
improved to be more specific, this commit refactors the function to
remove the regex entirely.

The new implementation uses indexOf and split to parse the lines,
which is safer and more performant, completely eliminating the risk of
Regular Expression Denial of Service (ReDoS) vulnerabilities.

[vs-code-engineering]

📬 CODENOTIFY

The following users are being notified based on files changed in this PR:

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Matched files:

• extensions/git/src/git.ts

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[lszomoru]

@aka76bm, please address the merge conflicts. Also the PR contains quite some files that are out of scope.