add secureexec file for getting around windows checking for a binary first in the current dir

vendor/github.com/cli/safeexec/README.md

@@ -0,0 +1,40 @@
+# safeexec
+
+A Go module that provides a safer alternative to `exec.LookPath()` on Windows.
+
+The following, relatively common approach to running external commands has a subtle vulnerability on Windows:
+```go
+import "os/exec"
+
+func gitStatus() error {
+    // On Windows, this will result in `.\git.exe` or `.\git.bat` being executed
+    // if either were found in the current working directory.
+    cmd := exec.Command("git", "status")
+    return cmd.Run()
+}
+```
+
+Searching the current directory (surprising behavior) before searching folders listed in the PATH environment variable (expected behavior) seems to be intended in Go and unlikely to be changed: https://github.com/golang/go/issues/38736
+
+Since Go does not provide a version of [`exec.LookPath()`](https://golang.org/pkg/os/exec/#LookPath) that only searches PATH and does not search the current working directory, this module provides a `LookPath` function that works consistently across platforms.
+
+Example use:
+```go
+import (
+    "os/exec"
+    "github.com/cli/safeexec"
+)
+
+func gitStatus() error {
+    gitBin, err := safeexec.LookPath("git")
+    if err != nil {
+        return err
+    }
+    cmd := exec.Command(gitBin, "status")
+    return cmd.Run()
+}
+```
+
+## TODO
+
+Ideally, this module would also provide `exec.Command()` and `exec.CommandContext()` equivalents that delegate to the patched version of `LookPath`. However, this doesn't seem possible since `LookPath` may return an error, while `exec.Command/CommandContext()` themselves do not return an error. In the standard library, the resulting `exec.Cmd` struct stores the LookPath error in a private field, but that functionality isn't available to us.