git_config: Stop using backslash to wrap lines

Unwrap one unnecessarily wrapped line, and use parentheses on
a wrapped condition instead of wrapping with backslashes.

Change-Id: I12679a0547dd822b15a6551e0f6c308239ff7b2d
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/254607
Reviewed-by: Mike Frysinger <vapier@google.com>
Tested-by: David Pursehouse <dpursehouse@collab.net>