cargo install --git requires manual version specification for prereleases

[jrobsonchase]

It appears that cargo is unable to locate a crate from a git repository if it has a version such as 0.2.7-alpha.0, as sccache currently does.

jchase@atlas sccache $ cargo install --git https://github.com/mozilla/sccache sccache
    Updating git repository `https://github.com/mozilla/sccache`
error: could not find `sccache` in https://github.com/mozilla/sccache    

Manually specifying the version allows it to actually find/build/install the crate though:

jchase@atlas sccache $ cargo install --git https://github.com/mozilla/sccache --force --version 0.2.7-alpha.0 sccache
    Updating git repository `https://github.com/mozilla/sccache`
  Installing sccache v0.2.7-alpha.0 (https://github.com/mozilla/sccache#bbff69f2)
...

My cargo version:

jchase@atlas cargo $ cargo --version
cargo 1.28.0-nightly (e2348c2db 2018-06-07)

[matklad]

Should be fixed by #5616