Re: https://github.com/rust-lang/rust is not bisectable

[Askar Safin <safinaskar@gmail.com>]

At some point that "bootstrap" word appeared, then at some point it
disappeared again. I'm trying to find a point where it has
disappeared. Consider this script:

==begin
#!/bin/bash

set -e
set -x

git bisect reset
git bisect start
git bisect good 7175c499ecc32cb3ff713be0bbac9fd12990a34e
git bisect bad 49c2279ef658d8732597c4da93897d84838f3df5

while :; do
  if grep -q bootstrap compiler/rustc_target/src/abi/mod.rs; then
    git bisect good
  else
    git bisect bad
  fi
  sleep 1
done
==end

Here is the output: https://paste.debian.net/1264444/ . As you can
see, we always choose "else git bisect bad". And we reach... initial
commit!

Well, technically speaking, this is correct: initial commit is
actually the first commit, where word "bootstrap" is not present in
compiler/rustc_target/src/abi/mod.rs . But this is not what I want! I
want to find commit, which is the first one, which doesn't have
"bootstrap" in compiler/rustc_target/src/abi/mod.rs, AND which is
descendant of good commit ( 7175c499ecc32cb3ff713be0bbac9fd12990a34e
).

Also, after failing with "git bisect", I did MANUAL BISECT!!!!!! I did
this: I run command "gitk --ancestry-path $GOOD..$BAD". Then I clicked
to some commit located nearly in the middle of history, then I copied
its ID. Then I switched to this commit and checked whether it is good.
Then I again run command "gitk --ancestry-path $GOOD..$BAD", this time
with a smaller interval. I did this several times and eventually I was
able to find the faulty commit! (Well, the result was not good enough:
this manual bisecting gave me some merge commit, so I still don't see
the actual commit. But it is still better than "git bisect": "git
bisect" simply gave me the initial commit.)

So, as you can see, manual bisecting (somewhat) works and "git bisect"
doesn't. This means that something is wrong with "git bisect". (Maybe
there is a way to make "git bisect" behave similar to that "gitk"
experiment above?)

Let me say this in other words: I want git-bisect to mark all commits,
which are not descendants of good, as good. (Here I assume bad is
descendant of good.)

-- 
Askar Safin