Improve test instructions to be more foolproof

Also put them in a place where they won't be overwritten.

Author: Byron

gix-diff/tests/README.md

@@ -0,0 +1,26 @@
+## How to run diff-slider tests
+
+The idea is to use https://github.com/mhagger/diff-slider-tools to create slider information for use to generate a test which invokes our own implementation and compares it to Git itself.
+Follow these instructions to set it up.
+
+1. DIFF_SLIDER_TOOLS=/your/anticipated/path/to/diff-slider-tools
+2. `git clone https://github.com/mhagger/diff-slider-tools $DIFF_SLIDER_TOOLS`
+3. `pushd $DIFF_SLIDER_TOOLS`
+4. Follow [these instructions](https://github.com/mhagger/diff-slider-tools/blob/b59ed13d7a2a6cfe14a8f79d434b6221cc8b04dd/README.md?plain=1#L122-L146) to     generate a file containing the slider information. Be sure to set the `repo` variable as it's used in later script invocations.
+   - Note that `get-corpus` must be run with `./get-corpus`.
+   - You can use an existing repository, for instance via `repo=git-human`, so there is no need to find your own repository to test.
+   - The script suite is very slow, and it's recommended to only set a range of commits, or use a small repository for testing.
+
+Finally, run the `internal-tools` program to turn that file into a fixture called `make_diff_for_sliders_repo.sh`.
+
+```shell
+# run inside `gitoxide`
+popd
+cargo run --package internal-tools -- \
+  create-diff-cases \
+    --sliders-file $DIFF_SLIDER_TOOLS/corpus/$repo.sliders \
+    --worktree-dir $DIFF_SLIDER_TOOLS/corpus/$repo.git/ \
+    --destination-dir gix-diff/tests/fixtures/
+```
+
+Finally, run `cargo test -p gix-diff-tests sliders -- --nocapture` to execute the actual tests to compare.

gix-diff/tests/fixtures/.gitignore

@@ -0,0 +1,2 @@
+# The slider assets that are needed for that slider fixture script to work and build the fixture.
+/assets/

gix-diff/tests/fixtures/generated-archives/.gitignore

@@ -0,0 +1,2 @@
+# The auto-generated sliders fixtures. For now it's experimental, but we may store it later once it's all working.
+/make_diff_for_sliders_repo.tar

gix-diff/tests/fixtures/make_diff_for_sliders_repo.sh

@@ -5,19 +5,4 @@ set -eu -o pipefail
 # `create-diff-cases` subcommand of `internal-tools`. The placeholder does
 # nothing except making the associated test trivially pass.
 #
-# ## Usage
-#
-# Follow these instructions to generate a file containing sliders:
-# https://github.com/mhagger/diff-slider-tools/blob/b59ed13d7a2a6cfe14a8f79d434b6221cc8b04dd/README.md?plain=1#L122-L146
-#
-# Run `create-diff-cases` to create the script in `gix-diff/tests/fixtures/`.
-# The script will be called `make_diff_for_sliders_repo.sh` and it will
-# overwrite the placeholder.
-#
-# ```
-# # run inside `gitoxide`
-# cargo run --package internal-tools -- create-diff-cases --sliders-file $PATH_TO_DIFF_SLIDER_TOOLS/corpus/git.sliders --worktree-dir $PATH_TO_DIFF_SLIDER_TOOLS/corpus/git.git/ --destination-dir gix-diff/tests/fixtures/
-# ```
-#
-# Run `cargo test sliders -- --nocapture` inside `gix-diff/tests` to run the
-# actual tests.
+# See the `gix-diff/tests/README.md` file for more information.