Add --msg-filename option

[asottile]

The --msg-filename option enables the following:

• Easier checking in a commit-msg hook without requiring a cat process.
• Easy integration with http://pre-commit.com

This is related to #38

[asottile]

@jorisroovers I've rebased this branch (just noticed the conflicts -- sorry!)

[asottile]

@jorisroovers any feedback on this patch?

[pbregener]

Can't you already do gitlint < commit_message.txt instead?

[asottile]

@pbregener that would require a shell

[jorisroovers]

Thanks for your interest in gitlint. I apologize it took me this look to get back to you - life and other projects get in the way sometimes :)

Definitely like the idea.
I think we could actually do something more generic here by calling the parameter --commit-message and supporting the @ syntax like many other tools do. Like so:

gitlint --commit-message "This is my commit message"
# Handy if you want to read in an env variable
gitlint --commit-message "$MESSAGE"

gitlint --commit-message @/path/to/my/commit/messagefile.txt

Thoughts?

Let me know if you still have time to work on this, if not, I'll see if I can squeeze it in.

[asottile]

I've literally never seen a tool accept an argument like that. It should really be two separate options (either --commit-msg ... or --msg-filename ...)

The wacky @ syntax isn't compatible with #38 either :(

[jorisroovers]

Here's one example: https://github.com/jakubroztocil/httpie#request-items

Admittedly, the syntax not widely popular - but I've seen it used elsewhere.

Anyways, I'm good with this then. I'll merge it in - I'll update the docs and integration tests later (appreciate any PRs related to that).

[asottile]

I think I already added some tests for ya :)

[jorisroovers]

integration tests in the https://github.com/jorisroovers/gitlint/tree/master/qa directory, but all good - I can do that later.

[asottile]

oh, I see -- sorry missed that :)