feat: make tournament script use git tags instead of branches

this makes more sense and is more convenient (branches can only be
checked out in one worktree)

contrib/fast-chess-tag.sh

@@ -1,21 +1,30 @@
 #!/bin/sh
 # Runs a fast-chess (https://github.com/Disservin/fastchess) tournament based
-# on two branches of the chess_inator
+# on two tags of the chess_inator (https://github.com/dogeystamp/chess_inator)
-# (https://github.com/dogeystamp/chess_inator) engine.
+# engine.
 #
 # Example usage:
 #
 #	 cd chess_tournaments
-#	 fast-chess-branch.sh quiescence no-quiescence -openings file=8moves_v3.pgn format=pgn order=random -each tc=300+0.1 -rounds 12 -repeat -concurrency 8 -recover -sprt elo0=0 elo1=10 alpha=0.05 beta=0.05
+#	 fast-chess-tag.sh quiescence no-quiescence -openings file=8moves_v3.pgn format=pgn order=random -each tc=300+0.1 -rounds 12 -repeat -concurrency 8 -recover -sprt elo0=0 elo1=10 alpha=0.05 beta=0.05
 #
-# Do not use `main` as a branch, or any other branch already checked out in
+# You need to be in a chess_inator Git repository to run this script. Ensure
-# another directory. You need to be in a chess_inator Git repository to run
+# that the repository you're in is a throw-away worktree. Create one using:
-# this script. Ensure that the repository you're in is a throw-away worktree.
-# Create one using
 #
 # 	git worktree add ../chess_tournaments
 #
 # inside the original chess_inator repo.
+#
+# To create a git tag at the current commit, run
+#
+# 	git tag [tag_name]
+#
+# or to optionally annotate the tag,
+#
+# 	git tag -a [tag_name]
+#
+# which will prompt for a message.
+#
 # Also, get an opening book from Stockfish's books:
 #
 # 	curl -O https://github.com/official-stockfish/books/raw/refs/heads/master/8moves_v3.pgn.zip
@@ -28,43 +37,45 @@
 # errors. The tournament automatically ends when a statistically significant
 # result is obtained.
 #
+# LOS stands for "likelihood of superiority", LLR "log likelihood ratio".
+#
 # By default, a PGN file will be exported with the games played, and the
 # fast-chess SPRT output will be appended. This comment may interfere with
 # importing the PGN. But Lichess will ignore it, so it's probably fine.
 
 set -e
 
-BRANCH1="$1"
+TAG1="$1"
-BRANCH2="$2"
+TAG2="$2"
 
 # if this line fails it's because you don't have enough arguments
 shift 2
 
-COMM1=$(git rev-parse --short "$BRANCH1")
+COMM1=$(git rev-parse --short "$TAG1")
-COMM2=$(git rev-parse --short "$BRANCH2")
+COMM2=$(git rev-parse --short "$TAG2")
 
 mkdir -p games
 
-PGN=games/"$BRANCH1"__"$BRANCH2".pgn
+PGN=games/"$TAG1"__"$TAG2".pgn
 
 rm -f engine1 engine2
 if [ -f "$PGN" ]; then
 	rm -i "$PGN"
 fi
 
-git switch "$BRANCH1"
+git checkout "$TAG1"
 cargo build --release
 cp target/release/chess_inator engine1
 
-git switch "$BRANCH2"
+git checkout "$TAG2"
 cargo build --release
 cp target/release/chess_inator engine2
 
 OUTPUT=$(mktemp)
 
 fastchess \
-	-engine cmd=engine1 name="c_i $BRANCH1 ($COMM1)" \
+	-engine cmd=engine1 name="c_i $TAG1 ($COMM1)" \
-	-engine cmd=engine2 name="c_i $BRANCH2 ($COMM2)" \
+	-engine cmd=engine2 name="c_i $TAG2 ($COMM2)" \
 	-pgnout file="$PGN" \
 	timeleft=true \
 	$@ \