built-in rebase --skip/--abort: clean up stale .git/<name> files

The scripted version of the rebase used to execute `git reset --hard`
when skipping or aborting. When we ported this to C, we did update the
worktree and some reflogs, but we failed to imitate `git reset --hard`'s
behavior regarding files in .git/ such as MERGE_HEAD.

Let's address this oversight.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

Author: dscho

builtin/rebase.c

@@ -22,6 +22,7 @@
 #include "wt-status.h"
 #include "revision.h"
 #include "rerere.h"
+#include "branch.h"
 
 static char const * const builtin_rebase_usage[] = {
 	N_("git rebase [-i] [options] [--exec <cmd>] [--onto <newbase>] "
@@ -1001,6 +1002,7 @@ int cmd_rebase(int argc, const char **argv, const char *prefix)
 
 		if (reset_head(NULL, "reset", NULL, 0, NULL, NULL) < 0)
 			die(_("could not discard worktree changes"));
+		remove_branch_state();
 		if (read_basic_state(&options))
 			exit(1);
 		goto run_rebase;
@@ -1018,6 +1020,7 @@ int cmd_rebase(int argc, const char **argv, const char *prefix)
 			       options.head_name, 0, NULL, NULL) < 0)
 			die(_("could not move back to %s"),
 			    oid_to_hex(&options.orig_head));
+		remove_branch_state();
 		ret = finish_rebase(&options);
 		goto cleanup;
 	}