[Dev] merge_arrow_pr.py script fails if head pointer can't be checked out

[asfimport]

Merge complete (local ref PR_TOOL_MERGE_PR_11241_MASTER). Push to apache? (y/n): y
Enumerating objects: 17, done.
Counting objects: 100% (17/17), done.
Delta compression using up to 32 threads
Compressing objects: 100% (8/8), done.
Writing objects: 100% (9/9), 1.98 KiB | 1.98 MiB/s, done.
Total 9 (delta 7), reused 0 (delta 0), pack-reused 0
remote: Resolving deltas: 100% (7/7), completed with 7 local objects.
To github.com:apache/arrow.git
   e5dc9d411..689a0b696  PR_TOOL_MERGE_PR_11241_MASTER -> master
Restoring head pointer to 6254bf77
error: pathspec '6254bf77' did not match any file(s) known to git
Command failed: ['git', 'checkout', '6254bf77']

Why does the script need to do anything that mutates the state of the clone?

Reporter: Phillip Cloud / @cpcloud

_{Note: This issue was originally created as ARROW-14154. Please see the migration documentation for further details.}

[nealrichardson]

@kou @raulcd @assignUser is this still valid? We use the GH API now so this shouldn't be happening.

[raulcd]

You are correct, that should not happen anymore the git checkout command was removed when we moved to the GH API: 01e4ad0