fix: unlink symlinks before copying git hooks

there was an issue where if a git hook with the same name existed
already, but was a symlink, ka-clone would overrwrite the link
destination’s contents rather than replacing the symlink!  apparently
this is what python’s `shutil.copyfile` does by design.

as a fix, manually unlink any symlinks first to be extra safe, then use
`shutil.copy` instead.

#devenvfixup

Auditors: csilvers, ethan, mopewa

bin/ka-clone

@@ -124,7 +124,10 @@ def _install_git_hook(template_name, hook_name):
     if not os.path.isdir(hooks_dir):
         os.makedirs(hooks_dir)
     dst = os.path.join(hooks_dir, hook_name)
-    shutil.copyfile(src, dst)
+    # if dst is a symlink, unlink (remove) it first, to avoid overwriting
+    if os.path.islink(dst):
+        os.unlink(dst)
+    shutil.copy(src, dst)
     os.chmod(dst, (os.stat(dst)).st_mode | 0111)  # ensure chmod +x