Fix `increment_path()` explicit file vs dir handling (#5523)

Resolves https://github.com/ultralytics/yolov5/pull/5341#issuecomment-961774729

Tests:
```python
import glob
import re
from pathlib import Path


def increment_path(path, exist_ok=False, sep='', mkdir=False):
# Increment file or directory path, i.e. runs/exp --> runs/exp{sep}2, runs/exp{sep}3, ... etc.
path = Path(path) # os-agnostic
if path.exists() and not exist_ok:
path, suffix = (path.with_suffix(''), path.suffix) if path.is_file() else (path, '')
dirs = glob.glob(f"{path}{sep}*") # similar paths
matches = [re.search(rf"%s{sep}(\d+)" % path.stem, d) for d in dirs]
i = [int(m.groups()[0]) for m in matches if m] # indices
n = max(i) + 1 if i else 2 # increment number
path = Path(f"{path}{sep}{n}{suffix}") # increment path
if mkdir:
path.mkdir(parents=True, exist_ok=True) # make directory
return path


print(increment_path('runs'))
print(increment_path('export.py'))
print(increment_path('abc.def.dir'))
print(increment_path('abc.def.file'))
```

utils/general.py

@@ -830,13 +830,12 @@ def increment_path(path, exist_ok=False, sep='', mkdir=False):
     # Increment file or directory path, i.e. runs/exp --> runs/exp{sep}2, runs/exp{sep}3, ... etc.
     path = Path(path)  # os-agnostic
     if path.exists() and not exist_ok:
-        suffix = path.suffix
-        path = path.with_suffix('')
+        path, suffix = (path.with_suffix(''), path.suffix) if path.is_file() else (path, '')
         dirs = glob.glob(f"{path}{sep}*")  # similar paths
         matches = [re.search(rf"%s{sep}(\d+)" % path.stem, d) for d in dirs]
         i = [int(m.groups()[0]) for m in matches if m]  # indices
         n = max(i) + 1 if i else 2  # increment number
-        path = Path(f"{path}{sep}{n}{suffix}")  # update path
+        path = Path(f"{path}{sep}{n}{suffix}")  # increment path
     if mkdir:
         path.mkdir(parents=True, exist_ok=True)  # make directory
     return path