Disable Git fsmonitor in virtual repositories

[tyrielv]

Summary

core.usebuiltinfsmonitor is considered deprecated in favor of core.fsmonitor which also supports setting the value to a hook. We will set both for backward git.exe compatibility.

• Set core.fsmonitor=false in required GVFS repository configuration.
• Mirror the setting in functional-test control repositories.

Validation

• dotnet build src\GVFS\GVFS.UnitTests\GVFS.UnitTests.csproj -c Debug --no-restore passed.
• Common unit-test filter ran, but the existing suite reported 133 environment-related failures in WorktreeEnlistmentTests and related setup paths.