cherry-pick: Only fetch if remote branch doesn't exist

Cherry-pick was updated to fetch if the branch doesn't
exist, but this could fetch unnecessarily if the remote
branch exists locally but namespaced in the remote.

Make this more clear with 3 paths:
- branch exists locally
- branch doesn't exist but remote version of it does (no fetch needed)
- neither of the above exists, will attempt to fetch

revup/cherry_pick.py

@@ -1,4 +1,5 @@
 import argparse
+import asyncio
 import logging
 
 from revup import git
@@ -14,11 +15,19 @@ async def find_branch_fetch_if_necessary(git_ctx: git.Git, branch_to_pick: str)
     Returns the ref for the local or remote branch
     """
     remote_branch_to_pick = git_ctx.ensure_branch_prefix(branch_to_pick)
-    branch_exists = await git_ctx.is_branch_or_commit(branch_to_pick)
-
-    if not branch_exists:
+    branch_exists, remote_branch_exists = asyncio.gather(
+        git_ctx.is_branch_or_commit(branch_to_pick),
+        git_ctx.is_branch_or_commit(remote_branch_to_pick),
+    )
+    if branch_exists:
+        return branch_to_pick
+    elif remote_branch_exists:
+        logging.info(f"Couldn't find '{branch_to_pick}', assuming '{remote_branch_to_pick}'")
+        return remote_branch_to_pick
+    else:
         logging.info(
-            f"Couldn't find '{branch_to_pick}', trying to fetch from remote '{git_ctx.remote_name}'"
+            f"Couldn't find '{branch_to_pick}', assuming '{remote_branch_to_pick}'"
+            " and trying to fetch from remote"
         )
 
         await git_ctx.git(
@@ -32,12 +41,9 @@ async def find_branch_fetch_if_necessary(git_ctx: git.Git, branch_to_pick: str)
         )
 
         if await git_ctx.is_branch_or_commit(remote_branch_to_pick):
-            logging.info(f"Found '{remote_branch_to_pick}'")
-            branch_to_pick = remote_branch_to_pick
+            return remote_branch_to_pick
         else:
-            raise RevupUsageException(f"Couldn't find ref '{branch_to_pick}'")
-
-    return branch_to_pick
+            raise RevupUsageException(f"Couldn't find ref '{branch_to_pick}' in local or remote!")
 
 
 async def main(args: argparse.Namespace, git_ctx: git.Git) -> int: