mingw: support spawning programs containing spaces in their names

The CreateProcessW() function does not really support spaces in its
first argument, lpApplicationName. But it supports passing NULL as
lpApplicationName, which makes it figure out the application from the
(possibly quoted) first argument of lpCommandLine.

Let's use that trick (if we are certain that the first argument matches
the executable's path) to support launching programs whose path contains
spaces.

This fixes https://github.com/git-for-windows/git/issue/692

Signed-off-by: Johannes Schindelin <[email protected]>

compat/mingw.c

@@ -1294,7 +1294,9 @@ static pid_t mingw_spawnve_fd(const char *cmd, const char **argv, char **deltaen
 	si.hStdError = winansi_get_osfhandle(fherr);
 
 	/* executables and the current directory don't support long paths */
-	if (xutftowcs_path(wcmd, cmd) < 0)
+	if (*argv && !strcmp(cmd, *argv))
+		wcmd[0] = L'\0';
+	else if (xutftowcs_path(wcmd, cmd) < 0)
 		return -1;
 	if (dir && xutftowcs_path(wdir, dir) < 0)
 		return -1;
@@ -1323,8 +1325,8 @@ static pid_t mingw_spawnve_fd(const char *cmd, const char **argv, char **deltaen
 	wenvblk = make_environment_block(deltaenv);
 
 	memset(&pi, 0, sizeof(pi));
-	ret = CreateProcessW(wcmd, wargs, NULL, NULL, TRUE, flags,
-		wenvblk, dir ? wdir : NULL, &si, &pi);
+	ret = CreateProcessW(*wcmd ? wcmd : NULL, wargs, NULL, NULL, TRUE,
+		flags, wenvblk, dir ? wdir : NULL, &si, &pi);
 
 	free(wenvblk);
 	free(wargs);