Move code to clean up after a branch change to branch.c

Signed-off-by: Daniel Barkalow <barkalow@iabervon.org>
diff --git a/builtin-reset.c b/builtin-reset.c
index 3bec06b..af0037e 100644
--- a/builtin-reset.c
+++ b/builtin-reset.c
@@ -16,6 +16,7 @@
 #include "diff.h"
 #include "diffcore.h"
 #include "tree.h"
+#include "branch.h"
 
 static const char builtin_reset_usage[] =
 "git-reset [--mixed | --soft | --hard] [-q] [<commit-ish>] [ [--] <paths>...]";
@@ -270,10 +271,7 @@
 		break;
 	}
 
-	unlink(git_path("MERGE_HEAD"));
-	unlink(git_path("rr-cache/MERGE_RR"));
-	unlink(git_path("MERGE_MSG"));
-	unlink(git_path("SQUASH_MSG"));
+	remove_branch_state();
 
 	free(reflog_action);