test: time rexdiff on large repositories, and add library benchmarks

.doc/dev.md

@@ -33,15 +33,18 @@ To run the tests against a real Rex too:
 
 ```sh
 REXDIFF_REX=$(which rex) go test ./test/
+REXDIFF_REX=$(which rex) test/e2e.sh
 ```
 
+`test/e2e.sh` also needs jq.
+
 ## Release
 
 ```sh
-gh release create v0.3.0
+gh release create vX.Y.Z
 ```
 
-GitHub Actions builds the binaries and updates the Homebrew cask.
+GitHub Actions runs the tests again, builds the binaries, and updates the Homebrew cask.
 
 ## Rules
 

.doc/test.md

@@ -9,6 +9,8 @@ rexdiff has library tests and black-box tests.
 - Each library is tested through its public API only, in `package x_test`.
 - A test that needs git runs real git in a temporary repository. `internal/testgit`
   hides your git config, so hooks and signing do not change the result.
+- A test uses the real limits of a library. A hook in `export_test.go` lowers a limit only
+  if the real size makes the test slow.
 - A test gives the library its own environment, with fake tools on its own `PATH`. No
   test changes the environment of the process, so all of them run in parallel.
 
@@ -17,12 +19,18 @@ rexdiff has library tests and black-box tests.
 - `tui` has no unit tests. The tests in `test/` build the binary and use it as you do.
 - Each test types keys and clicks through a terminal emulator, and reads the screen.
 - A test waits for the screen or for a file, never for a fixed time.
-- One test waits for the log: `TestLastTurn` waits for the line `msg="turn start"` in
-  the debug log.
+- Some tests wait for a line in the debug log, for example `msg="turn start"`.
 - The scripts in `test/testdata/script` test the commands, for example `install`, with
   testscript.
 - Shell scripts stand in for rex, gh, the clipboard, and the editors. The fake rex
   prints output recorded from a real Rex.
-- `TestRealRex` runs rexdiff in a real Rex, to find a fake that no longer matches Rex.
+- `test/e2e.sh` runs rexdiff in a real Rex, one step at a time. It walks through the user
+  guide, hostile file names, tiny and huge blocks, boxes open while the block resizes, and
+  a long review. Each step waits until the screen shows a whole frame. The script also
+  checks that rex still prints JSON in the shapes that the fake rex records. Run it with
+  `REXDIFF_REX=$(which rex) test/e2e.sh`.
 - The black-box tests run one at a time, because each one is a real program in a
   terminal.
+- The perf tests in `test/perf_test.go` time the program on large repositories. They run
+  only if `REXDIFF_PERF` names a directory for the repositories. The libraries have Go
+  benchmarks: `go test -bench . ./diff ./vcs ./internal/tree`.