internal/vcs/git: add FindNearestCommit to get commit nearest to a point in time

Created by: slimsag

Stacked on top of #18712

For building historical data of search insights, we need a way to locate the commit in a repository closest to some given point in time so that we can search it using a repo:<repo>@<rev> filter and determine the number of results that existed around that point in time. This adds the ability for us to determine which commit is most appropriate for a given time range.

Signed-off-by: Stephen Gutekanst [email protected]