check main branch by default when cargo install --git executed

[non-descriptive]

Due to recent changes default branch of new git projects will be set to main instead of master, so it is probably good idea to check both default branch names. Attempt to install project with main default branch with just cargo install --git currently will fail with

error: failed to find branch `master`

                                     Caused by:
                                                 cannot locate remote-tracking branch 'origin/master'; class=Reference (4); code=NotFound (-3)

Current workaround requires explicitly pass --branch main to cargo.

last tested on
cargo 1.47.0 (f3c7e066a 2020-08-28)

[ehuss]

Thanks for the report. Closing as a duplicate of #3517.