Fix bug handling absolute source paths with no --src-prefix -- just output to…

Fix bug handling absolute source paths with no --src-prefix -- just output to the same directory in this case.

c++/src/capnp/compiler/capnp.c++

@@ -268,6 +268,11 @@ public:
     // Strip redundant "./" prefixes to make src-prefix matching more lenient.
     while (file.startsWith("./")) {
       file = file.slice(2);
+
+      // Remove redundant slashes as well (e.g. ".////foo" -> "foo").
+      while (file.startsWith("/")) {
+        file = file.slice(1);
+      }
     }
 
     if (!compilerConstructed) {

c++/src/capnp/compiler/capnpc-c++.c++

@@ -1967,10 +1967,12 @@ private:
   }
 
   void writeFile(kj::StringPtr filename, const kj::StringTree& text) {
+    if (!filename.startsWith("/")) {
       KJ_IF_MAYBE(slashpos, filename.findLast('/')) {
         // Make the parent dir.
         makeDirectory(kj::str(filename.slice(0, *slashpos)));
       }
+    }
 
     int fd;
     KJ_SYSCALL(fd = open(filename.cStr(), O_CREAT | O_WRONLY | O_TRUNC, 0666), filename);