Add GitInfo

[bep]

This commit adds a GitInfo object to Page if EnableGitInfo is set.

It then also sets Lastmod for the given Page to the author date provided by Git.

The Git integrations should be fairly performant, but it adds "some time" to the build, somewhat depending on the Git history size.

If you want, you can run without during development and turn it on when deploying to the live server: hugo --enableGitInfo.

Fixes #2102

[bep]

This has some input:

http://stackoverflow.com/questions/9417634/list-last-commit-dates-for-a-large-number-of-files-quickly

[bep]

This should work:

git -C /Users/bep/go/src/github.com/spf13/hugo/ log --name-only --format=format:"%t|%aN|%aE|%ai|%s" HEAD

[bep]

I have updated the PR. There are still some small holes, but it is complete enough to know that it is doable.

So if other could chime in with their opinions about:

1. Is this a good idea? (I know I want it)
2. Does this implementation make sense?
3. Does setting Lastmod make sense?

/cc @spf13

[moorereason]

Haven't tested this, but I like the idea. 👍

Is the AbbreviatedHash necessary? That could be derived later from the Hash if you needed it.

[bep]

Is the AbbreviatedHash necessary?

You get it for free, and that is the value I want to show on my pages (if any); it's not possible to derive it from the Hash from inside of a Hugo template.

[moorereason]

{{ substr .GitInfo.Hash 0 -7 }}

[bep]

{{ substr .GitInfo.Hash 0 -7 }}

Which assumes 7 is a constant value.

[moorereason]

My understanding was that it's configurable (via the core.abbrev setting). Defaults to 7. If you change the setting in your repo, you could update your template to match.

I just thought we could save memory allocs, but it's not that much of a win to drop it. You win. :)

[bep]

You kind of got a point, but it is simplest to let Git and the Git config win ... The memory loss is ... not much. Maybe Git-Junio will invent some smart "sliding abbreviated" hashes in the future ... Which is only length 3 for small repos ...

[bep]

Note to self: Maybe add a CLI flag for this as well, to make it easier to switch it off when in server mode etc.

[bep]

This is ready now. It needs some docs, but that will have to wait.

/cc @digitalcraftsman @moorereason @spf13 and gang.

[github-actions]

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