repl: complete if all matches share prefix

There's gotta be a nicer way to do this
(without resorting to making a tree or something)
but this seems to do the job.

src/nix/repl.cc

@@ -131,6 +131,32 @@ static char * completionCallback(char * s, int *match) {
     auto *res = strdup(possible.begin()->c_str() + strlen(s));
     if (!res) throw Error("allocation failure");
     return res;
+  } else if (possible.size() > 1) {
+    size_t startLen = strlen(s);
+    size_t len = startLen;
+    while (true) {
+      bool same = true;
+      try {
+        char c = possible.begin()->at(len);
+        for (auto &p : possible) {
+          if (p.at(len) != c) {
+            same = false;
+            break;
+          }
+        }
+      } catch (std::out_of_range &) {
+        same = false;
+      }
+      if (!same)
+        break;
+      ++len;
+    }
+    if (len > startLen) {
+      *match = 1;
+      auto *res = strdup(std::string(*possible.begin(), startLen, len-startLen).c_str());
+      if (!res) throw Error("allocation failure");
+      return res;
+    }
   }
 
   *match = 0;