added helper function to get modification time from git

[JensTimmerman]

fixes #265

[zeuswpi-bot]

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[JensTimmerman]

fbc76e8 shows:

[JensTimmerman]

804c0b1 shows

[JensTimmerman]

as mentioned in #96

fetching the modification date of a single file is antithetical to the way git works
since git does not track per-file history. Fetching the date of a single
file is a Slow Operation because it requires scanning global history
looking for a commit in which the desired file was changed.

[werthen]

@mcbloch There's still an issue with this fix. A lot of posts now show an incorrect writing date, such as the following:

I think this should be reverted and reopened.

[JensTimmerman]

I think just removing the “created at” part of the pr I made was not a good idea.
It might fix the merge issue but won’t show the correct created date.

I can look into this later when I have some time.

Is there a documented way to add test to this repo? So a test can pick up a regression like this?

[werthen]

No custom tests yet, but should be easy enough to add using nanoc's checks: https://nanoc.ws/doc/testing/

[redfast00]

@JensTimmerman maybe use the first date the file was created instead of the last time?

[JensTimmerman]

@redfast00 can this be discussed in #265 ? I have asked a few questions on how you want to see edge cases handled in that issue

I believe the time the blog was merged makes more sense than the time a file with that name was created? since that is the original question in that issue