Use git-last-modified(1) for ListLastCommitsForTree RPC

Toon Claesrequested to merge
toon-git-last-modified into master

Avoid a bunch of N+1 git subprocesses to fill in the ListLastCommitsForTreeResponse, and instead use git-last-modified(1) which provides everything in one go.

Issue: #6727 (closed)

Edited by Toon Claes

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