feat: make zlib-ng-compat a default feature flag

This way it can be turned off via `cargo build --no-deafult-flags` which
is required to link against a shared `libgit2`.

This PR also bumps `git2-rs` to link against the `1.6` branch of `libgit2`

Cargo.toml

@@ -16,13 +16,14 @@ include = [
     "README.md",
     "LICENSE"
 ]
+rust-version = "1.60" # for `--features`
 
 [dependencies]
 anyhow = { version = "1.0.68", features = ["backtrace"] }
 clap = { version = "4.1.2", features = ["derive"] }
 ctrlc = "3.2.4"
 dialoguer = "0.10.3"
-git2 = { version = "0.16.1", default-features = false, features = [ "zlib-ng-compat" ] }
+git2 = { version = "0.17.1", default-features = false }
 handlebars = "4.3.6"
 regex = "1.7.1"
 semver = "1.0.16"
@@ -38,6 +39,10 @@ clap = { version = "4.1.2", features = ["derive"] }
 clap_complete = "4.1.1"
 semver = "1.0.16"
 
+[features]
+default = ["zlib-ng-compat"]
+zlib-ng-compat = ["git2/zlib-ng-compat"]
+
 [package.metadata.deb]
 depends = ""
 extended-description = """\

README.md

@@ -20,7 +20,15 @@ It provides the following commands:
 
 `cargo install convco`
 
-This build depends on `git2` with the `zlib-ng-compat` feature. It requires `cmake`.
+## Building from source
+
+Rust 1.60 or newer is required.
+
+Building with `cargo` depends on `git2` and `cmake` due to linking with `zlib-ng`.
+You can optionally disable this by changing the defaults for a build:
+```console
+$ cargo build --no-default-features
+```
 
 ## Configuration