build(macos): don't use ssh feature for git2 dependency

Using the `ssh` feature induces a dependency on `openssl`. On my Macbook, this produced the following linking error:

```
  = note: Undefined symbols for architecture x86_64:
            "_OPENSSL_init_ssl", referenced from:
                std::sync::once::Once::call_once::_$u7b$$u7b$closure$u7d$$u7d$::he73245cadb6fb5f7 (.llvm.1763620816825828942) in libopenssl_sys-e129a1c9f9682ba1.rlib(openssl_sys-e129a1c9f9682ba1.openssl_sys.dn7psuzk-cgu.6.rcgu.o)
                core::ops::function::FnOnce::call_once$u7b$$u7b$vtable.shim$u7d$$u7d$::h2d3710fadd6753f9 (.llvm.1763620816825828942) in libopenssl_sys-e129a1c9f9682ba1.rlib(openssl_sys-e129a1c9f9682ba1.openssl_sys.dn7psuzk-cgu.6.rcgu.o)
          ld: symbol(s) not found for architecture x86_64
          clang: error: linker command failed with exit code 1 (use -v to see invocation)
```

I tried the following:

* Running `brew install [email protected]` as per the instructions at https://docs.rs/openssl/0.10.33/openssl/
* Setting the environment variables mentioned in `brew info openssl`
* Setting `LIBGIT2_SYS_USE_PKG_CONFIG=1` as mentioned in rust-lang/git2-rs#229

None of these worked. So I'm disabling the `ssh` feature for now. If `git-branchless` needs the `ssh` feature in the future (for example, if we add a Github integration and want it to pull/push directly), then we'll have to resolve the issue properly.

It's possible my particular issue is due to running Homebrew on a too-old version of macOS, and that it might be resolved by an OS upgrade and re-installing my Homebrew packages.

Author: arxanas

Cargo.lock

@@ -22,7 +22,7 @@ assert_cmd = "1.0.3"
 console = "0.14.0"
 cursive = {version = "0.16.3", features = ["crossterm-backend"]}
 fn-error-context = "0.1.1"
-git2 = "0.13.17"
+git2 = {version = "0.13.17", default-features = false}
 lazy_static = "1.4.0"
 log = "0.4.14"
 regex = "1.4.4"