git: speed up extraction of git children call

- don't use all refs,
- break early if we found the children

vcsserver/remote/git.py

@@ -944,17 +944,22 b' class GitRemote(RemoteBase):'
         cache_on, context_uid, repo_id = self._cache_on(wire)
         region = self._region(wire)
 
+        head = self.head(wire)
+
         @region.conditional_cache_on_arguments(condition=cache_on)
         def _children(_repo_id, _commit_id):
+
             output, __ = self.run_git_command(
-                wire, ['rev-list', '--all', '--children'])
+                wire, ['rev-list', '--all', '--children', f'{commit_id}^..{head}'])
 
             child_ids = []
-            pat = re.compile(r'^%s' % commit_id)
-            for l in output.splitlines():
-                if pat.match(l):
-                    found_ids = l.split(' ')[1:]
+            pat = re.compile(r'^{}'.format(commit_id))
+            for line in output.splitlines():
+                line = safe_str(line)
+                if pat.match(line):
+                    found_ids = line.split(' ')[1:]
                     child_ids.extend(found_ids)
+                    break
 
             return child_ids
         return _children(repo_id, commit_id)