tabs, git diff and git apply

[martin rudalics]

Some three lines appearing in my window.c as

      if (WINDOWP (w->contents))
        cont = foreach_window_1 (XWINDOW (w->contents), fn, user_data);
      else

appear in *shell* after git diff as

+      if (WINDOWP (w->contents))
+       cont = foreach_window_1 (XWINDOW (w->contents), fn, user_data);
+      else

Note the extra space after the "+" in the second line and the off-by-one
indentation.  A subsequent git apply then complains about

/home/martin/temp/master..changes-2020-11-28.diff:3847: space before tab in 
indent.
        cont = foreach_window_1 (XWINDOW (w->contents), fn, user_data);

Now my sources contain many lines where text is preceded by exactly one
tab on its line but this is the only one that produces such a failure.
Does anyone have an explanation for that behavior?

Thanks in advance, martin