Link Git debug trace vars in useful_git_commands.md

Link upstream docs with all Git debug traces & env variables

doc/topics/git/useful_git_commands.md

@@ -167,6 +167,14 @@ With HTTPS:
 GIT_TRACE_PACKET=1 GIT_TRACE=2 GIT_CURL_VERBOSE=1 git clone <url>
 ```
 
+### Debugging with Git embedded traces
+
+Git includes a complete set of [traces for debugging Git commands](https://git-scm.com/book/en/v2/Git-Internals-Environment-Variables#_debugging), for example:
+
+- `GIT_TRACE_PERFORMANCE=1`: enables tracing of performance data, showing how long each particular `git` invocation takes.
+- `GIT_TRACE_SETUP=1`: enables tracing of what `git` is discovering about the repository and environment it’s interacting with.
+- `GIT_TRACE_PACKET=1`: enables packet-level tracing for network operations.
+
 ## Rebasing
 
 ### Rebase your branch onto master