doc: improve subtree check instructions

test/lint/README.md

@@ -23,6 +23,12 @@ maintained:
 * for `src/crypto/ctaes`: https://github.com/bitcoin-core/ctaes.git (branch master)
 * for `src/crc32c`: https://github.com/google/crc32c.git (branch master)
 
+To do so, add the upstream repository as remote:
+
+```
+git remote add --fetch secp256k1 https://github.com/bitcoin-core/secp256k1.git
+```
+
 Usage: `git-subtree-check.sh DIR (COMMIT)`
 
 `COMMIT` may be omitted, in which case `HEAD` is used.

test/lint/git-subtree-check.sh

@@ -81,7 +81,7 @@ fi
 
 # get the tree in the subtree commit referred to
 if [ "d$(git cat-file -t $rev 2>/dev/null)" != dcommit ]; then
-    echo "subtree commit $rev unavailable: cannot compare" >&2
+    echo "subtree commit $rev unavailable: cannot compare. Did you add and fetch the remote?" >&2
     exit
 fi
 tree_subtree=$(git show -s --format="%T" $rev)