git_config: fix error message command output

str() inside an f-string is redundant.  Actually join the array together
as a string to show a proper error message, and include the full command
line, not the extra options passed in.

Before:
error.GitError: git config ('--null', '--list'): fatal: unable to read config file 'xxx': No such file or directory

After:
error.GitError: git config --system --includes --null --list: fatal: unable to read config file 'xxx': No such file or directory

Change-Id: I8983389aa2e0de7808991e73e636b77810f04c4b
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/582341
Commit-Queue: Mike Frysinger <vapier@google.com>
Tested-by: Mike Frysinger <vapier@google.com>
Reviewed-by: Gavin Mak <gavinmak@google.com>

git_config.py

@@ -438,7 +438,7 @@ class GitConfig:
         if p.Wait() == 0:
             return p.stdout
         else:
-            raise GitError(f"git config {str(args)}: {p.stderr}")
+            raise GitError(f"git {' '.join(command)}: {p.stderr}")
 
 
 class RepoConfig(GitConfig):