Move RepoHook class from project.py file to dedicated file

The project.py file is huge and contains multiple
classes. By moving it to seperate class files the code
becomes more readable and maintainable.

Signed-off-by: Remy Bohmer <github@bohmer.net>
Change-Id: Ida9d99d31751d627ae1ea0373418080696d2e14b
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/281293
Reviewed-by: Mike Frysinger <vapier@google.com>
Tested-by: Remy Bohmer <linux@bohmer.net>

subcmds/upload.py

@@ -24,7 +24,7 @@ from editor import Editor
 from error import HookError, UploadError
 from git_command import GitCommand
 from git_refs import R_HEADS
-from project import RepoHook
+from hooks import RepoHook
 
 from pyversion import is_python3
 if not is_python3():