refactor(memline): distinguish mutating uses of ml_get_buf()

ml_get_buf() takes a third parameters to indicate whether the
caller wants to mutate the memline data in place. However
the vast majority of the call sites is using this function
just to specify a buffer but without any mutation. This makes
it harder to grep for the places which actually perform mutation.

Solution: Remove the bool param from ml_get_buf(). it now works
like ml_get() except for a non-current buffer. Add a new
ml_get_buf_mut() function for the mutating use-case, which can
be grepped along with the other ml_replace() etc functions which
can modify the memline.

src/nvim/indent.c

@@ -378,10 +378,7 @@ int get_indent_lnum(linenr_T lnum)
 // "buf".
 int get_indent_buf(buf_T *buf, linenr_T lnum)
 {
-  return get_indent_str_vtab(ml_get_buf(buf, lnum, false),
-                             buf->b_p_ts,
-                             buf->b_p_vts_array,
-                             false);
+  return get_indent_str_vtab(ml_get_buf(buf, lnum), buf->b_p_ts, buf->b_p_vts_array, false);
 }
 
 /// Count the size (in window cells) of the indent in line "ptr", with