Add endpoint to the GPT: compare commits

Closes #79 (closed)

API doc: Compare branches, tags or commits

Looking through the Kibana logs, it appears that compare branches request is not very popular, but compare branches is called often, so the original issue to add compare branches was changed to add compare commits.

(+) Small change to error log if no message is found, then print the response body error content, like 404 Not Found.

Problem:

• Used commit_sha and commit_sha_signed commits have 192 differences which lead to slow response time

NAME	RPS	RPS RESULT	TTFB AVG	TTFB P90	REQ STATUS	RESULT
api_v4_projects_repository_compare_commits	200/s	12.93/s (>64.00/s)	10210.10ms	13111.02ms (<3000ms)	100.00% (>95%)	FAILED²

Solved

Test results above weren't very realistic, as we compared commits from different branches with a very big amount of changes. Here are more accurate results of comparison of the 2 commits on the same branch with 4 commits and 30 diffs:

Results summary:

* Environment:                10k
* Environment Version:        12.7.0-pre `3a67004c0ae`
* Option:                     60s_200rps
* Date:                       2020-02-13
* Run Time:                   1m 3.92s (Start: 16:00:14 UTC, End: 16:01:18 UTC)
* GPT Version:                v1.1.2

█ Overall Results Score: 95.33%

NAME                                       | RPS   | RPS RESULT           | TTFB AVG | TTFB P90          | REQ STATUS     | RESULT         
-------------------------------------------|-------|----------------------|----------|-------------------|----------------|----------------
api_v4_projects_repository_compare_commits | 200/s | 190.65/s (>160.00/s) | 340.93ms | 358.89ms (<600ms) | 100.00% (>95%) | Passed