[FEATURE] Improve git commit time lookup performance with memoization

[RichardForshaw]

Is your feature request related to a problem? Please describe.
Based on my mkdocs configuration, the publish date for my blog articles are generated multiple times. As the list of articles grows, the build time becomes longer and longer, and git.log is already a bit slow...

Describe the solution you'd like
This can be sped up with memoisation - using the functools.cache decorator (from python 3.9) or similar on the git_get_commit_timestamp function means that the subsequent 2 calls on the same file return instantaneously. A brief test has already cut my build time by over 50%.

Describe alternatives you've considered
I've looked into speeding up git.log but can't find anything useful.

[liang2kl]

Good suggestion! Can you create a PR for this?