Fix some lint warnings (#189)

Remove unused imports or variable, and apply suggestions from https://adamj.eu/tech/2019/09/12/how-i-import-pythons-datetime-module/.

src/common/git.py

@@ -1,6 +1,6 @@
 from hashlib import sha1
 from pathlib import Path
-from subprocess import run, PIPE
+from subprocess import run
 
 
 class Git:
@@ -18,7 +18,7 @@ class Git:
             child = run(f"git {cmd}", capture_output=True, timeout=300, check=True, shell=True, cwd=self.repo_dir)
             return child.stdout.decode("utf-8").strip().split("\n")
         except ChildProcessError as ex:
-            raise RuntimeError(f"Failed to run '{git_command}': {ex}")
+            raise RuntimeError(f"Failed to run '{cmd}': {ex}")
 
     def setup(self, bare: bool = False):
         """Creates the repository path and runs: