Check if the HEAD commit is publicly available on GitHub. If it is, set it in the database (#95043)

## About The Pull Request

TGS provides two SHA's to the game. The first is the closest commit on
the tracked branch (`master`) the second is the true revision of the
game. The latter may be equal to the first, but not when test merges are
active.

We normally set the database `commit_hash` to the first because we can't
guarantee the latter is on GitHub. TGS **does** try to push it there
however (i.e.
https://github.com/tgstation/tgstation/commit/b7c18d2bdb6b091255716fa801113be4d73be987)
provided it has proper credentials.

This PR will ping the HEAD SHA on GitHub to see if it exists there. If
it does, it'll be set as the round's `commit_hash`.

## Why It's Good For The Game

Allows statbus to link to the EXACT commit of the round which includes
test merges. Currently it lies when test merges are active by linking
the `master` commit.

## Changelog

🆑
server: If TGS is configured to push test merge commits to a publicly
accessible GitHub, the exact game revision will be set as the
`round`.`commit_hash` in the database.
/🆑

@nfreader

---------

Co-authored-by: Jordan Dominion <Cyberboss@users.noreply.github.com>

code/datums/helper_datums/getrev.dm

@@ -41,6 +41,26 @@
 
 	return msg.Join("\n")
 
+/datum/getrev/proc/GetDatabaseCommitSha()
+	. = originmastercommit
+	if (commit == .)
+		return .
+
+	var/gh_url = CONFIG_GET(string/githuburl)
+	if(!gh_url)
+		return .
+
+	var/datum/http_request/request = new()
+	request.prepare(RUSTG_HTTP_METHOD_GET, "[gh_url]/commit/[commit]", "", "")
+	request.begin_async()
+	UNTIL(request.is_complete())
+	var/datum/http_response/response = request.into_response()
+
+	if (response.status_code >= 200 && response.status_code < 300)
+		return commit
+
+	return .
+
 /datum/getrev/proc/GetTestMergeInfo(header = TRUE)
 	if(!testmerge.len)
 		return ""