mingw: work around incorrect standard handles

For some reason, when being called via TortoiseGit the standard handles
can take on the value (HANDLE)-2 (which is not a legal value, according
to the documentation). Nevertheless, CreateProcess() works with
hStdInput set to this value. But our new code to restrict which file
handles get inherited by spawned processes apparently does *not* work
with such values, erroring out with `ERROR_INVALID_PARAMETER`.

Let's just disallow "negative" handles, and hopefully this will work
around the issue.

This addresses git-for-windows#1481

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

Author: dscho

compat/mingw.c

@@ -1662,12 +1662,12 @@ static pid_t mingw_spawnve_fd(const char *cmd, const char **argv, char **deltaen
 	si.StartupInfo.hStdError = winansi_get_osfhandle(fherr);
 
 	/* The list of handles cannot contain duplicates */
-	if (si.StartupInfo.hStdInput != INVALID_HANDLE_VALUE)
+	if ((intptr_t)si.StartupInfo.hStdInput >= 0)
 		stdhandles[stdhandles_count++] = si.StartupInfo.hStdInput;
-	if (si.StartupInfo.hStdOutput != INVALID_HANDLE_VALUE &&
+	if ((intptr_t)si.StartupInfo.hStdOutput >= 0 &&
 	    si.StartupInfo.hStdOutput != si.StartupInfo.hStdInput)
 		stdhandles[stdhandles_count++] = si.StartupInfo.hStdOutput;
-	if (si.StartupInfo.hStdError != INVALID_HANDLE_VALUE &&
+	if ((intptr_t)si.StartupInfo.hStdError >= 0 &&
 	    si.StartupInfo.hStdError != si.StartupInfo.hStdInput &&
 	    si.StartupInfo.hStdError != si.StartupInfo.hStdOutput)
 		stdhandles[stdhandles_count++] = si.StartupInfo.hStdError;