Eliminate conhost.exe from git.exe and hooks process launches

GVFS launches git.exe processes with CreateNoWindow=true (or CREATE_NO_WINDOW
in native code). Despite the name, this flag tells Windows to create a NEW
hidden console for the child process, which allocates a conhost.exe instance.
For frequent, small git operations (e.g. during prefetch), the per-process
conhost creation/teardown overhead is disproportionately large.

Fix: Set CreateNoWindow=false in GitProcess.cs. With UseShellExecute=false
and stdout/stderr redirected to pipes, the child process inherits the parent's
console state. Since GVFS.Mount runs as a service with no console, the child
gets no console and no conhost.

ProcessHelper.cs is left unchanged — it is used by gvfs.hooks.exe and gvfs.exe
(CLI) which run with a console, where the existing behavior is appropriate.

For native code (GitHooksLoader.cpp), replace CREATE_NO_WINDOW with
DETACHED_PROCESS, which truly detaches from any console without allocating
a new one.

Reference: ToolLocationTransformer PR 15071136 (same pattern)
Linked: ADO #62385282

Author: Nathan Baird

GVFS/GVFS.Common/Git/GitProcess.cs

@@ -824,7 +824,7 @@ public Process GetGitProcess(string command, string workingDirectory, string dot
             processInfo.RedirectStandardOutput = true;
             processInfo.RedirectStandardError = redirectStandardError;
             processInfo.WindowStyle = ProcessWindowStyle.Hidden;
-            processInfo.CreateNoWindow = true;
+            processInfo.CreateNoWindow = false;
 
             processInfo.StandardOutputEncoding = UTF8NoBOM;
             processInfo.StandardErrorEncoding = UTF8NoBOM;

GVFS/GitHooksLoader/GitHooksLoader.cpp

@@ -129,7 +129,7 @@ int ExecuteHook(const std::wstring &applicationName, wchar_t *hookName, int argc
         /* Git disallows stdin from hooks */
         si.dwFlags = STARTF_USESTDHANDLES;
 
-        creationFlags |= CREATE_NO_WINDOW;
+        creationFlags |= DETACHED_PROCESS;
     }
 
     ZeroMemory(&pi, sizeof(pi));