secp-sys: update README for new vendoring script

Also a couple minor tweaks to the vendoring script itself.

Author: apoelstra

secp256k1-sys/README.md

@@ -20,11 +20,12 @@ This prefix ensures that no symbol collision can happen:
 To update the vendored sources, use the `vendor-libsecp.sh` script:
 
 ```
-$ ./vendor-libsecp.sh depend <version-code> <rev>
+$ ./vendor-libsecp.sh <rev>
 ```
 
-- Where `<version-code>` is the secp256k1-sys version number underscored: `0_1_2`.
-- Where `<rev>` is the git revision of libsecp256k1 to checkout.
+Where `<rev>` is the git revision of libsecp256k1 to checkout. If you do not
+specify a revision, the script will simply clone the repo and use whatever
+revision the default branch is pointing to.
 
 
 ## Linking to external symbols

secp256k1-sys/depend/secp256k1-HEAD-revision.txt

@@ -1,2 +1,2 @@
-# This file was automatically created by ./vendor-libsecp.sh
+# This file was automatically created by vendor-libsecp.sh
 21ffe4b22a9683cf24ae0763359e401d1284cc7a

secp256k1-sys/vendor-libsecp.sh

@@ -2,7 +2,11 @@
 set -e
 
 # Set default variables
-: "${SECP_VENDOR_GIT_ROOT:=$(git rev-parse --show-toplevel)}"
+if [ -z "$SECP_VENDOR_GIT_ROOT" ]; then
+    SECP_VENDOR_GIT_ROOT="$(git rev-parse --show-toplevel)"
+else
+    SECP_VENDOR_GIT_ROOT="$(realpath "$SECP_VENDOR_GIT_ROOT")"
+fi
 SECP_SYS="$SECP_VENDOR_GIT_ROOT"/secp256k1-sys
 DEFAULT_VERSION_CODE=$(grep version "$SECP_SYS/Cargo.toml" | sed 's/\./_/g' | sed 's/.*"\(.*\)".*/\1/')
 DEFAULT_DEPEND_DIR="$SECP_SYS/depend"
@@ -83,7 +87,7 @@ rm -rf .git/ || true
 popd
 
 # Record revision
-echo "# This file was automatically created by $0" > ./secp256k1-HEAD-revision.txt
+echo "# This file was automatically created by $(basename "$0")" > ./secp256k1-HEAD-revision.txt
 echo "$SOURCE_REV" >> ./secp256k1-HEAD-revision.txt
 
 # Patch source files