builtin-commit: remove .git/SQUASH_MSG upon successful commit

After doing a merge --squash, and commit afterwards, the commit message
template SQUASH_MSG in the git directory is not removed, which means that
the content of SQUASH_MSG is used as default commit message for all
subsequent commits.  So have git commit remove the file SQUASH_MSG from
the git directory upon a successful commit.

The problem was discovered by Frédéric Brière, reported through
 http://bugs.debian.org/464656Signed-off-by: NGerrit Pape <pape@smarden.org>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

builtin-commit.c

@@ -929,6 +929,7 @@ int cmd_commit(int argc, const char **argv, const char *prefix)
 
 	unlink(git_path("MERGE_HEAD"));
 	unlink(git_path("MERGE_MSG"));
+	unlink(git_path("SQUASH_MSG"));
 
 	if (commit_index_files())
 		die ("Repository has been updated, but unable to write\n"