check-abi: Minor UI improvements

tools/check-abi.sh

@@ -6,14 +6,16 @@ default_base_version="$(git describe --match "v*.*.*" --abbrev=0)"
 default_new_version="HEAD"
 
 display_help_and_exit() {
-    echo "Usage: $0 <base_ver> <new_ver>"
+    echo "Usage: $0 [<base_ver> [<new_ver>]]"
     echo ""
     echo "Description: This script uses the ABI Compliance Checker tool to determine if the ABI"
     echo "             of a new version of libsecp256k1 has changed in a backward-incompatible way."
     echo ""
     echo "Options:"
-    echo "  base_ver      Specify the base version (default: $default_base_version)"
-    echo "  new_ver       Specify the new version (default: $default_new_version)"
+    echo "  base_ver      Specify the base version as a git commit-ish"
+    echo "                  (default: most recent reachable tag matching \"v.*.*\", currently \"$default_base_version\")"
+    echo "  new_ver       Specify the new version as a git commit-ish"
+    echo "                  (default: $default_new_version)"
     echo "  -h, --help    Display this help message"
     exit 0
 }
@@ -23,9 +25,11 @@ if [ "$#" -eq 0 ]; then
     new_version="$default_new_version"
 elif [ "$#" -eq 1 ] && { [ "$1" = "-h" ] || [ "$1" = "--help" ]; }; then
     display_help_and_exit
-elif [ "$#" -eq 2 ]; then
+elif [ "$#" -eq 1 ] || [ "$#" -eq 2 ]; then
     base_version="$1"
-    new_version="$2"
+    if [ "$#" -eq 2 ]; then
+        new_version="$2"
+    fi
 else
     echo "Invalid usage. See help:"
     echo ""