Stop calling ftell() in a loop.

ftell() must go ask the kernel for the file offset, in case
someone knew the underlying file descriptor number and seeked it.
Thus, we can save a couple hundred thousand syscalls by just
caching the offset and maintaining it ourselves.

This cuts another ~170ms off a no-op Chromium build.

src/deps_log.cc

@@ -186,15 +186,13 @@ LoadStatus DepsLog::Load(const string& path, State* state, string* err) {
     return LOAD_SUCCESS;
   }
 
-  long offset;
+  long offset = ftell(f);
   bool read_failed = false;
   int unique_dep_record_count = 0;
   int total_dep_record_count = 0;
   for (;;) {
-    offset = ftell(f);
-
     unsigned size;
-    if (fread(&size, 4, 1, f) < 1) {
+    if (fread(&size, sizeof(size), 1, f) < 1) {
       if (!feof(f))
         read_failed = true;
       break;
@@ -206,6 +204,7 @@ LoadStatus DepsLog::Load(const string& path, State* state, string* err) {
       read_failed = true;
       break;
     }
+    offset += size + sizeof(size);
 
     if (is_deps) {
       if ((size % 4) != 0) {