Use the full path for noshebang'd scripts

If you make a script called `foo` somewhere in $PATH, and did not give
it a shebang, this would end up calling

    sh foo

instead of

    sh /usr/bin/foo

which might not match up.

Especially if the path is e.g. `--version` or `-` that would end up
being misinterpreted *by sh*.

So instead we simply pass the actual_cmd to sh, because we need it
anyway to get it to fail to execute before.

src/exec.cpp

@@ -150,6 +150,7 @@ bool is_thompson_shell_script(const char *path) {
     // This function never returns, so we take certain liberties with constness.
     auto envv = const_cast<char **>(cenvv);
     auto argv = const_cast<char **>(cargv);
+    auto cmd2 = const_cast<char *>(actual_cmd);
 
     execve(actual_cmd, argv, envv);
     err = errno;
@@ -169,6 +170,9 @@ bool is_thompson_shell_script(const char *path) {
             char interp[] = _PATH_BSHELL;
             argv2[0] = interp;
             std::copy_n(argv, 1 + nargs, &argv2[1]);  // +1 to copy terminating nullptr
+            // The command to call should use the full path,
+            // not what we would pass as argv0.
+            argv2[1] = cmd2;
             execve(_PATH_BSHELL, argv2, envv);
         }
     }

src/postfork.cpp

@@ -350,7 +350,11 @@ maybe_t<pid_t> posix_spawner_t::spawn(const char *cmd, char *const argv[], char
             std::vector<char *> argv2;
             char interp[] = _PATH_BSHELL;
             argv2.push_back(interp);
-            for (size_t i = 0; argv[i] != nullptr; i++) {
+            // The command to call should use the full path,
+            // not what we would pass as argv0.
+            std::string cmd2 = cmd;
+            argv2.push_back(&cmd2[0]);
+            for (size_t i = 1; argv[i] != nullptr; i++) {
                 argv2.push_back(argv[i]);
             }
             argv2.push_back(nullptr);

tests/checks/noshebang.fish

@@ -80,3 +80,13 @@ runfile
 
 #CHECK: 126
 #CHECKERR: exec: {{.*}}
+
+echo 'echo foo' >./-
+sleep 0.1
+chmod +x ./-
+set PATH ./ $PATH
+sleep 0.1
+-
+#CHECK: foo
+echo $status
+#CHECK: 0