fast-export: avoid using unnecessary language in a code comment

In an ongoing effort to avoid non-inclusive language, let's avoid using
the branch name "master" in a code comment.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/builtin/fast-export.c b/builtin/fast-export.c
index 1b8fca3..5527135 100644
--- a/builtin/fast-export.c
+++ b/builtin/fast-export.c
@@ -1026,7 +1026,7 @@
 				/*
 				 * Getting here means we have a commit which
 				 * was excluded by a negative refspec (e.g.
-				 * fast-export ^master master).  If we are
+				 * fast-export ^HEAD HEAD).  If we are
 				 * referencing excluded commits, set the ref
 				 * to the exact commit.  Otherwise, the user
 				 * wants the branch exported but every commit