Split GitCommitProvider and move LoadCommits from prebuild to file build

[OsmondJiang]

1. split git commit provider and git commit cache provider
2. combine repository provider and git commit provider
3. save git commit cache at the end of the build
4. remove contribution provider load, make it as inner call of build page, no need to load commits before build
5. make the git commit provider lifetime consistent with build, be able to be re-used everywhere during one build
6. Fixes git commit provider is not thread safe #3700

[superyyrrzz]

[yufeih]

What is the peak memory usage and build time after this change for azure-docs-pr? This is performance sensitive code so I want to know some numbers.

[OsmondJiang]

i will test it using my local machine

The peak memory is 5.5 GB, bigger than the preview way because now the loadCommits are inner call of build page, not a preview step, so the "tree" cache keeps in the memory until the build finished:

The build files time:

• with cache:(under 1G)
  • build files: 44s
  • convert legacy 1:22s
• without cache:(5.5G)
  • build files: 2:52s
  • legacy 1:30

And we find an interesting problem, the gitCommitProvider will not be GC until build although it's an variable of buildFiles method, it seems referenced by ParallelUtility.Foreach, @yufeih, need your input here, please reference above picture

[yufeih]

The build files time:

• with cache:(under 1G)

  • build files: 44s
  • convert legacy 1:22s

• without cache:(5.5G)

  • build files: 2:52s
  • legacy 1:30

How does this compare to the result before this change?

[yufeih]

👍