commit: Wait for the git-log(1) process to exit in FindCommits

This is a re-open MR to investigate failed rail test.

The reverted MR is here !6507 (merged)

In this commit, we add a new wait logic to wait for git-log(1) command to finish;

Previously we did not Wait() for the git-log(1) process to exit in FindCommits, causing us to fail to include its execution time in the command details and not log any errors it returned.

We differentiate benign errors from real errors.

• Benign errors are caused by terminating the stream since response limit is reached; while real errors are caused by git log command failures, such as timeout, OOM, or calling a non-existing commit.
• If there is not commits returned, a RPC NotFound error is returned.

Closes #5671 (closed)