Upstream basic implementation of git-last-modified(1)

Stolee and Taylor from GitHub informed us that they've been running a custom builtin git-blame-tree and asked for our help to upstream it to mainline Git. Stolee will put out a patch for the library code, and we can help in the capacity of reviewing it as well as helping with the CLI portion. They mentioned that Junio was not a fan of the original semantics of their CLI, so we can think through how to make the semantics nicer.

This will allow us to improve the performance of the ListLastCommitsForTree RPC drastically: gitaly#1337 (closed). Currently the implementation is to run git-log(1) for every tree entry. With this change, we can just call blame-tree on a tree and get the commits that touched it.

This issue aims to upstream the initial version. Additional features and improvements are planned to be added through Optimize annotating files with the last commit ... (&17844)