Help messages not shown correctly when post command hook is present

[tyrielv]

Repro steps:
In a repository where post-command hook is configured (such as a VFSForGit clone)

1. Configure autocorrect:
   git config help.autocorrect=prompt
2. Run an misspelled command. For example,
   git statuss

Expected output:

WARNING: You called a Git command named 'statuss', which does not exist.
Run 'status' instead [y/N]?

Actual output:

failed to run command 'statuss': No error

I think the problem is that run_post_command_hook needs to save and restore errno, similar to how handle_alias does it. Otherwise, the errno from the failed command is overwritten by the successful errno from postcommand hook before the help autocorrect has a chance to examine it.