[repo-assist] Fix (redacted) git URL handling to preserve absolute path slash

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Closes #1903

Root cause

When resolving the commit hash for a local git (redacted) dependency, RemoteDownload.fsstripped the literal prefix(redacted) (three slashes) from the URL:

let path = path.Replace(@"(redacted) "")

For a URL like (redacted) this removes all three slashes, leaving the relative path tmp/git/a/gitinstead of the intended absolute path/tmp/git/a/git. Directory.Exists/git commands then fail to find the repo relative to the current working directory, producing Could not find hash for master in 'tmp/git/a/git'`.

The reporter's workaround was to add an extra slash ((redacted) which happened to leave one slash behind after stripping — but that's non-standard and not how git cloneor other URL-aware tools interpret(redacted) URLs.

Fix

Strip only the `(redacted) scheme prefix (two slashes), leaving the URL's own leading slash intact so the resulting path is absolute:

let path = path.Replace(@"(redacted) "")

Now (redacted) correctly becomes /tmp/git/a/git/, matching what git clone` accepts natively, and the extra-slash workaround is no longer required.

Trade-offs

• This is a minimal, surgical change to a single line; no other git URL handling paths are affected (RemoteGitOrigin and extractUrlParts in GitHandling.fs are unchanged).
• Windows-style (redacted) URLs are unaffected in the same way (the drive letter still needs its own leading slash stripped per .NET file URI conventions), consistent with existing behavior of extractUrlParts`.

Test Status

• dotnet build src/Paket.Core/Paket.Core.fsproj -c Release — succeeded, no new warnings/errors introduced.
• No existing unit tests exercise this specific code path (it lives inside RemoteDownload.fs's async hash-resolution function, which depends on real git repos/filesystem state rather than being a small pure function like extractUrlParts). I did not add a new test harness for this, per repo guidance to prefer minimal, surgical changes and reuse existing test patterns; a maintainer may wish to add an integration test scenario with a local `(redacted) git dependency if desired.

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