Fix increment_dir to use run_xxx for logdir (#1058)

* Fix increment_dir to use run_xxx for logdir 

Rerunning train.py with logdir in the form of `run_xxx` causes index slicing in
```
n = max([int(x[len(dir):x.find('_') if '_' in Path(x).name else None]) for x in d]) + 1  # increment
```
to underflow.

* Replace find with rfind

utils/general.py

@@ -948,7 +948,7 @@ def increment_dir(dir, comment=''):
     dir = str(Path(dir))  # os-agnostic
     d = sorted(glob.glob(dir + '*'))  # directories
     if len(d):
-        n = max([int(x[len(dir):x.find('_') if '_' in x else None]) for x in d]) + 1  # increment
+        n = max([int(x[len(dir):x.rfind('_') if '_' in Path(x).name else None]) for x in d]) + 1  # increment
     return dir + str(n) + ('_' + comment if comment else '')