chore: optimize/harden the license script

This script is still a little awkward, but it's now 30x faster and more
correct:

1. It will auto-update the copyright year.
2. It will apply the correct author (Filecoin Core Devs).
3. It will work without committing files first.
4. It will insert lines in the correct order.

scripts/add_license.sh

@@ -2,40 +2,35 @@
 #
 # Checks if the source code contains required license and adds it if necessary.
 # Returns 1 if there was a missing license, 0 otherwise.
-COPYRIGHT_TXT=$(dirname $0)/copyright.txt
-
-# Any year is fine. We can update the year as a single PR in all files that have it up to last year.
-PAT_PL=".*// Copyright 2021-202\d Protocol Labs.*"
-# Files that were taken from ChainSafe and not modified can be left as-is.
-PAT_CS=".*// Copyright 2019-2022 ChainSafe Systems.*"
-PAT_SPDX="/*// SPDX-License-Identifier: Apache-2.0, MIT.*"
-
-# Look at enough lines so that we can include multiple copyright holders.
-LINES=4
-
+YEAR="$(date '+%Y')"
 ret=0
+set -e -x
 
 # Look for files without headers.
-for file in $(git grep --cached -Il '' -- '*.rs'); do
-  header=$(head -$LINES "$file")
-	if ! echo "$header" | grep -q -P "$PAT_SPDX"; then
-		echo "$file was missing header"
-		cat $COPYRIGHT_TXT "$file" > temp
-		mv temp "$file"
-		ret=1
-	fi
-done
+while read -r file; do
+    echo "$file was missing header"
+    sed -i "$file" -f - <<EOF
+1i\
+// SPDX-License-Identifier: Apache-2.0, MIT'
+EOF
+    ret=1
+done < <(git grep -IL "^// SPDX-License-Identifier:" -- '*.rs')
 
-# Look for changes that don't have the new copyright holder.
-for file in $(git diff --diff-filter=d --name-only master -- '*.rs'); do
-  header=$(head -$LINES "$file")
-	if ! echo "$header" | grep -q -P "$PAT_PL"; then
-		echo "$file was missing Protocol Labs"
-		head -1 $COPYRIGHT_TXT > temp
-		cat "$file" >> temp
-		mv temp "$file"
-		ret=1
-	fi
-done
+while read -r file; do
+    if grep -q "^// Copyright \([0-9]\+-\)\?${YEAR} Filecoin Core Devs" "$file"; then
+        continue
+    fi
+    ret=1
+    if grep -q "^// Copyright [0-9]\+\(-[0-9]\+\)\? Filecoin Core Devs" "$file"; then
+        # Update the copyright line if available.
+        script="s|^// Copyright \([0-9]\+\)\(-[0-9]\+\)\? Filecoin Core Devs|// Copyright \1-${YEAR} Filecoin Core Devs|"
+    else
+        # Otherwise, add the new one _AFTER_ the SDPX header.
+        script="/^\/\/ SPDX-License-Identifier: /a\
+// Copyright ${YEAR} Filecoin Core Devs
+"
+    fi
+    sed -i "$file" -e "$script"
+done < <(git diff --diff-filter=DM --name-only --merge-base master -- '*.rs')
 
 exit $ret