write index without output path to memory only.

That way the actual performance can be measured more easily, removing IO
entirely.

gitoxide-core/src/repository/index.rs

@@ -8,7 +8,6 @@ pub fn from_tree(
     index_path: Option<PathBuf>,
     force: bool,
     repo: git::Repository,
-    mut out: impl std::io::Write,
 ) -> anyhow::Result<()> {
     spec.push("^{tree}");
     let spec = git::path::os_str_into_bstr(&spec)?;
@@ -29,6 +28,7 @@ pub fn from_tree(
         }
         None => {
             let index = git::index::File::from_state(index, std::path::PathBuf::new());
+            let mut out = Vec::with_capacity(512 * 1024);
             index.write_to(&mut out, options)?;
         }
     }

src/plumbing/main.rs

@@ -814,8 +814,8 @@ pub fn main() -> Result<()> {
                 progress,
                 progress_keep_open,
                 None,
-                move |_progress, out, _err| {
-                    core::repository::index::from_tree(spec, index_output_path, force, repository(Mode::Strict)?, out)
+                move |_progress, _out, _err| {
+                    core::repository::index::from_tree(spec, index_output_path, force, repository(Mode::Strict)?)
                 },
             ),
         },

src/plumbing/options/mod.rs

@@ -345,7 +345,8 @@ pub mod index {
             #[clap(long, short = 'f')]
             force: bool,
             /// Path to the index file to be written.
-            /// If none is given it will be written to stdout, avoiding to overwrite the repository index just yet.
+            /// If none is given it will be kept in memory only as a way to measure performance. One day we will probably write the index
+            /// back by default, but that requires us to write more of the index to work.
             #[clap(long, short = 'i')]
             index_output_path: Option<PathBuf>,
             /// A revspec that points to the to generate the index from.