This diff adds a new command to arcanist `arc scripted-diff` to verify
that the current commit is compliant with the application of the shell
script contained in the commit message.
This command is also run as part of `arc diff`.
The shell script should be contained between the following two markers
`-BEGIN/END VERIFY SCRIPT-`.
It is inspired by core PR10189.
Workflow:
- Extract the script from the commit message.
- Checkout the base commit.
- Run the script.
- Compare the working tree output with the working tree from the
scripted-diff commmit.
- Reset the working tree and checkout back to the scipted-diff commit.