diff: munmap() file contents before running external diff

When running an external diff from, say, a diff tool, it is safe to
assume that we want to write the files in question. On Windows, that
means that there cannot be any other process holding an open handle to
said files.

So let's make sure that `git diff` itself is not holding any open handle
to the files in question.

This fixes #1315

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

diff.c

@@ -3094,6 +3094,10 @@ static void run_external_diff(const char *pgm,
 	argv_array_pushf(&env, "GIT_DIFF_PATH_COUNTER=%d", ++o->diff_path_counter);
 	argv_array_pushf(&env, "GIT_DIFF_PATH_TOTAL=%d", q->nr);
 
+	if (one->should_munmap)
+		diff_free_filespec_data(one);
+	if (two->should_munmap)
+		diff_free_filespec_data(two);
 	if (run_command_v_opt_cd_env(argv.argv, RUN_USING_SHELL, NULL, env.argv))
 		die(_("external diff died, stopping at %s"), name);