Invoking `x` from a different directory seems to interpret build targets in a surprising way

[RalfJung]

Command used

git clone https://github.com/rust-lang/rust
mkdir sibling
cd sibling
touch miri
../rust/x check miri

Expected behaviour

This should build Miri, just like cd ../rust && ./x check miri does.

Actual behaviour

ERROR: no `check` rules matched [/home/r/src/rust/tmp/sibling/miri]

This only happens when I do the touch miri to actually create a file with that name. It seems like when I pass a path that actually exists relative to my current directory to x, then it will turn this into an absolute path and then try to find a build target for that path which of course cannot work.

HEAD

2f4dfc7