prepare-commit-msg: Add missing tabs to Fixes: and Event:

Although these line up when prefixed with "# ", once the comment prefix
is deleted they no longer do. Add an extra tab to each to compensate,
just like "Obtained from:".

tools/tools/git/hooks/prepare-commit-msg

@@ -48,11 +48,11 @@ $(awk '1;/^#$/{exit}' $1)
 # Tested by:	<If someone else tested the change.>
 # Approved by:	<If you needed approval for this commit.>
 # Obtained from:	<If the change is from a third party.>
-# Fixes:	<Short hash and title line of commit fixed by this change>
+# Fixes:		<Short hash and title line of commit fixed by this change>
 # MFC after:	<N [day[s]|week[s]|month[s]].  Request a reminder email>
 # Relnotes:	<Set to 'yes' for mention in release notes.>
 # Security:	<Vulnerability reference (one per line) or description.>
-# Event:	<If this was done at a conference.>
+# Event:		<If this was done at a conference.>
 # Sponsored by:	<If the change was sponsored by an organization.>
 # Pull Request:	<https://github.com/freebsd/<repo>/pull/###>
 # Differential Revision:	<https://reviews.freebsd.org/D###>