Ignore leading comments and empty lines when parsing commit via stdin

[dpc]

Is your feature request related to a problem? Please describe.
I just implemented commit-msg git hook linting with convco: https://github.com/fedimint/fedimint/pull/1573/files

I had to do some extra work, because the commit message can actually be prefixed with newlines and comment lines which git will happily throw away before turning the whole message into a actual commit, but convco will complain about.

Describe the solution you'd like
Not sure if should be the default behavior altogether, or just when parsing from stdin, or maybe an option, but it would be easier if convco could take care of it (ignore leading newlines and comments).

[hdevalke]

convco does this in the commit subcommand. And it looks like this is documented in git:

The commit-msg hook takes one parameter, which again is the path to a temporary file that contains the commit message written by the developer.

as this repository follows conventional commits, comments and extra newlines should be seen as an error, so i think the best way is to add a flag --ignore-newlines-and-comments

[hdevalke]

The behaviour of git can depend on the --cleanup mode.

[dpc]

-ignore-newlines-and-comments gets long and without "leading" a bit incorrect, so maybe --cleanup instead as well? :D

[hdevalke]

I used --strip other modes can be implemented later.

[hdevalke]

fixed in #116