Improve check on git-svnrevert, better error message

When the commit is not in the tree at all, find-rev returns 0
and prints an empty string. We need to catch that problem too,
when trying to revert.

Adding a list of possible problems, so that you can easily and
quickly correct without having to edit the script again.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@237516 91177308-0d34-0410-b5e6-96231b3b80d8

utils/git-svn/git-svnrevert

@@ -14,8 +14,13 @@ fi
 
 COMMIT=$1
 OTHER=$(git svn find-rev "$COMMIT")
-if [ $? -ne 0 ]; then
+if [ $? -ne 0 ] || [ "$OTHER" = "" ]; then
     echo "Error! Could not find an svn/git revision for commit $COMMIT!"
+    echo
+    echo "Possible problems are:"
+    echo "  * Your revision number ($COMMIT) is wrong"
+    echo "  * This tree is not up to date (before that commit)"
+    echo "  * This commit in in another three (llvm, clang, compiler-rt, etc)"
     exit 1
 fi