mingw: avoid spawning processes with a \\?\-prefixed executable

The thing is: Executables on Windows may inspect their own path via
_pgmptr (see https://msdn.microsoft.com/en-us/library/tza1y5f7.aspx)
e.g. to figure out the location of related files.

Git, for example, needs to figure out where the libexec/git-core/
directory is with its supporting Unix shell and Perl scripts (and for
well-intentioned backwards-compatibility, even dashed forms of the
builtins).

These programs, however, may very well be unaware that they were
possibly called with the \\?\ prefix, and as a consequence, they may
very well be unable to handle it.

One prominent example is MSYS2's gdb (MINGW version). The symptom is:

	$ C:/msys64/mingw64/bin/gdb.exe --help
	Could not find platform independent libraries <prefix>
	Could not find platform dependent libraries <exec_prefix>
	Consider setting $PYTHONHOME to <prefix>[:<exec_prefix>]
	ImportError: No module named site

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

win32/process.c

@@ -129,6 +129,21 @@ static intptr_t mingw_spawnve(int mode,
 		return -1;
 	}
 
+	/*
+	 * It can be easily verified that _wspawnve() has no problems with a
+	 * wcmd that has the \\?\ prefix and it may be a long path.
+	 *
+	 * However, some programs inspect their own absolute path, e.g. to
+	 * infer the location of related files (think: git.exe and its exec
+	 * path), and not all of these programs handle a \\?\ prefix well, e.g.
+	 * MSYS2's gdb (MINGW version).
+	 *
+	 * So let's skip the prefix when possible (i.e. when the path fits
+	 * within PATH_MAX minus some wiggling room).
+	 */
+	if (!wcsncmp(wcmd, L"\\\\?\\", 4) && wcslen(wcmd) < PATH_MAX)
+		wcmd += 4;
+
 	/*
 	 * We cannot use _P_WAIT here because we need to kill spawned processes
 	 * if we're killed, and _P_WAIT does not let us.