build: switch back to gix/max-performance-safe feature by default

Using `gix/max-performance` requires `cmake` as a build-time dependency, which could be a significant barrier for contributors (including existing ones, like me, who already work on jj but didn't have `cmake` installed thus far).

This commit switches back to using `gix/max-performance-safe`, which doesn't have the `cmake` dependency, and adds `gix/max-performance` behind the `gix-max-performance` feature for `jj-lib`.

It also adds `gix-max-performance` to the `packaging` feature group, since I'm assuming that packagers will want maximum performance, and are more likely to have `cmake` at hand.

Tested with

```
$ cargo build --workspace
$ cargo build --workspace --features packaging
```

(and the `--features packaging` build failed until I installed `cmake`)

Cargo.toml

@@ -61,7 +61,7 @@ gix = { version = "0.68.0", default-features = false, features = [
     "attributes",
     "blob-diff",
     "index",
-    "max-performance",
+    "max-performance-safe",
 ] }
 glob = "0.3.1"
 hashbrown = { version = "0.15.2", default-features = false, features = ["inline-more"] }

cli/Cargo.toml

@@ -111,7 +111,7 @@ jj-cli = { path = ".", features = ["test-fakes"], default-features = false }
 [features]
 default = ["watchman"]
 bench = ["dep:criterion"]
-packaging = []
+packaging = ["jj-lib/gix-max-performance"]
 test-fakes = ["jj-lib/testing"]
 vendored-openssl = ["git2/vendored-openssl", "jj-lib/vendored-openssl"]
 watchman = ["jj-lib/watchman"]

lib/Cargo.toml

@@ -98,6 +98,12 @@ tokio = { workspace = true, features = ["full"] }
 [features]
 default = ["git"]
 git = ["dep:git2", "dep:gix"]
+gix-max-performance = [
+    # Requires `cmake` as a build dependency.
+    # Note that this feature is different from `gix/max-performance-safe`.
+    # See: https://docs.rs/gix/latest/gix/#performance
+    "gix/max-performance",
+]
 vendored-openssl = ["git2/vendored-openssl"]
 watchman = ["dep:tokio", "dep:watchman_client"]
 testing = ["git"]