mfc-candidates: search by committer only, not author

When both --author and --committer are specified, `git log` requires
both to match.  Search only by committer, as it is typically the FreeBSD
committer who will perform the MFC.

Reviewed by:	jhb
Sponsored by:	The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D41964

tools/tools/git/mfc-candidates.sh

@@ -118,7 +118,7 @@ fi
 authorarg=
 if [ -n "$author" ]; then
 	# Match user ID in the email portion of author or committer
-	authorarg="--author <${author}@ --committer <${author}@"
+	authorarg="--committer <${author}@"
 fi
 
 # Commits in from_branch after branch point