End the tabs versus space war
Vim, 37, 34, 33, 32 bytes

:%s/\t/    /g|%norm ^hr/hv0r*r/


Try it online!

Note that this requires a trailing carriage return (enter) in vim, although not in the online interpreter.

This uses the V interpreter because it's backwards compatible. A very straightforward solution. 

Here's a gif that lets you see the solution happen in real time. This uses a slightly older version, and I added some extra keystrokes to make it run slower so you can see what happens:



And here is the explanation of how it works:

:%s/\t/    /g           "Replace every tab with 4 spaces
|                       "AND
%norm                   "On every line:
      ^                 "  Move to the first non-whitespace char
       h                "  Move one character to the left. If there is none, the command will end here.
         r/             "  Replace it with a slash
           h            "  Move to the left
            v0          "  Visually select everything until the first column
              r*        "  Replace this selection with asterisks
                r/      "  Replace the first character with a slash