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

Resolves ultralytics/yolov5#5341 (comment)

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