Git: Reduce PATH searches for executable

This reduces executions in BranchModel::updateUpstreamStatus (which is
called for each local branch with a tracking branch) from 3s to ~700ms
on Windows.

Change-Id: I92651ba8f37987bef49a80b46963964ae8cacb3c
Reviewed-by: Jarek Kobus <jaroslaw.kobus@qt.io>

src/plugins/git/gitsettings.h

@@ -40,6 +40,9 @@ public:
     Utils::BoolAspect refLogShowDate;
     Utils::BoolAspect instantBlame;
 
+    mutable Utils::FilePath resolvedBinPath;
+    mutable bool tryResolve = true;
+
     Utils::FilePath gitExecutable(bool *ok = nullptr, QString *errorMessage = nullptr) const;
 };