Clone: surface the real clone failure instead of a misleading InvalidDataException

[ShiningMassXAcc]

Summary

gvfs clone <url> <dst> --branch <nonexistent-branch> printed a misleading
downstream exception instead of the real, actionable failure. The console showed:

Cannot clone @ c:\s\osbr: System.IO.InvalidDataException: Couldn't open repo at C:\s\osbr\src: failed to resolve path 'C:\s\osbr\src': The system cannot find the file specified.
   at GVFS.Common.Git.LibGit2Repo..ctor(ITracer, String)
   ...
   at GVFS.CommandLine.CloneVerb.Execute()

…while the real error was only in the clone log, one line above an even-more-downstream warning:

[Error]   {"ErrorMessage":"Remote branch official/br_current_directes_dev not found in upstream origin"}
[Warning] {"WarningMessage":"Couldn't open repo at C:\s\osbr\src: failed to resolve path ..."}

Root cause

In CloneVerb.Execute() the LibGit2RepoInvoker for the enlistment src folder
was constructed unconditionally, even after cloneResult.Success == false. On a
failed clone the src working directory was never created, so LibGit2Repo's
constructor logged a Couldn't open repo at ...\src warning and threw an
InvalidDataException. That exception escaped the using (tracer) scope, bypassing
the existing else branch that prints Error: <real message>, and was caught by the
generic catch (Exception) handler that printed the misleading Cannot clone @ ...: InvalidDataException: Couldn't open repo message.

Fix

Guard the LibGit2RepoInvoker construction behind cloneResult.Success (and
initialize trustPackIndexes = false, since it is only read on the success path).
Failed clones now fall through to the existing else branch, which already prints
the real error:

Cannot clone @ c:\s\osbr
Error: Remote branch official/br_current_directes_dev not found in upstream origin

Tests

Added CloneTests.CloneWithNonExistentBranchFailsWithBranchError, which clones with a
non-existent --branch and asserts:

• exit code is the generic error (3),
• output contains Remote branch ... not found in upstream origin,
• output does not contain Couldn't open repo / InvalidDataException.

Validation

• dotnet build GVFS/GVFS/GVFS.csproj -c Debug — succeeded (0 errors).
• GVFS.FunctionalTests C# compiles; the new test exercises the fixed path (requires
  the full functional-test harness to execute).

Tracked by ADO Task 63117974 (parent Deliverable 60994157 — "Git reliability for razzle").

[tyrielv]

I suspect this is a duplicate of #2065

[ShiningMassXAcc]

Confirmed duplicate of #2065 — thanks @tyrielv. #2065 targets the same root cause in CloneVerb.Execute() (the unconditional LibGit2RepoInvoker construction on a failed clone) and gates the trustPackIndexes lookup on cloneResult.Success, so the branch-not-found case handled here now also falls through to the real Error: Remote branch ... not found in upstream origin message. #2065 additionally fixes the null-enlistment NRE and applies the same cleanup to PrefetchVerb/Hooks/Mount. Closing in favor of #2065.