Wrong cargo config resolution when using manifest-path

[LegNeato]

When using --manifest-path, cargo appears to be searching for a .cargo/config starting at the directory cargo build was RUN FROM, rather than the manifest-path location.

Directory structure to repro

• .cargo/
  • config => file containing source config pointing at ./vendor1
• vendor1/ => does not have serde vendored
• foo/
  • bar/
    • mycrate/ => the crate I am building
      • .cargo
        • config => file containing source config pointing at ./vendor2
    • Cargo.toml => Standard toml file for mycrate
    • vendor2/ => has serde vendored

Steps to repro

If I run cargo from a different directory and point manifest-path at mycrate/Cargo.toml, it traverses from pwd and finds the top-level .cargo rather than the one in mycrate.

$ cd foo/bar;
$ cargo build --frozen --release --manifest-path ./mycrate/Cargo.toml
error: no matching package named `serde` found (required by `mycrate`)
location searched: registry https://github.com/rust-lang/crates.io-index
version required: = 0.9.11

If I run the same command from mycrate, everything works:

$ cd mycrate
$ cargo build --frozen --release
   Compiling ring v0.6.3
   Compiling winapi v0.2.8
   Compiling language-tags v0.2.2
   Compiling regex-syntax v0.4.0
   Compiling security-framework-sys v0.1.10
   # ....
$

[alexcrichton]

I believe this is a dupe of #2930, but thanks for the report!