builtin-am: refuse to apply patches if index is dirty

Since d1c5f2a (Add git-am, applymbox replacement., 2005-10-07), git-am
will refuse to apply patches if the index is dirty. Re-implement this
behavior in builtin/am.c.

Signed-off-by: Paul Tan <pyokagan@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/am.c

@@ -14,6 +14,8 @@
 #include "cache-tree.h"
 #include "refs.h"
 #include "commit.h"
+#include "diff.h"
+#include "diffcore.h"
 
 /**
  * Returns 1 if the file is empty or does not exist, 0 otherwise.
@@ -564,6 +566,43 @@ static void refresh_and_write_cache(void)
 		die(_("unable to write index file"));
 }
 
+/**
+ * Returns 1 if the index differs from HEAD, 0 otherwise. When on an unborn
+ * branch, returns 1 if there are entries in the index, 0 otherwise. If an
+ * strbuf is provided, the space-separated list of files that differ will be
+ * appended to it.
+ */
+static int index_has_changes(struct strbuf *sb)
+{
+	unsigned char head[GIT_SHA1_RAWSZ];
+	int i;
+
+	if (!get_sha1_tree("HEAD", head)) {
+		struct diff_options opt;
+
+		diff_setup(&opt);
+		DIFF_OPT_SET(&opt, EXIT_WITH_STATUS);
+		if (!sb)
+			DIFF_OPT_SET(&opt, QUICK);
+		do_diff_cache(head, &opt);
+		diffcore_std(&opt);
+		for (i = 0; sb && i < diff_queued_diff.nr; i++) {
+			if (i)
+				strbuf_addch(sb, ' ');
+			strbuf_addstr(sb, diff_queued_diff.queue[i]->two->path);
+		}
+		diff_flush(&opt);
+		return DIFF_OPT_TST(&opt, HAS_CHANGES) != 0;
+	} else {
+		for (i = 0; sb && i < active_nr; i++) {
+			if (i)
+				strbuf_addch(sb, ' ');
+			strbuf_addstr(sb, active_cache[i]->name);
+		}
+		return !!active_nr;
+	}
+}
+
 /**
  * Parses `mail` using git-mailinfo, extracting its patch and authorship info.
  * state->msg will be set to the patch message. state->author_name,
@@ -726,9 +765,15 @@ static void do_commit(const struct am_state *state)
 static void am_run(struct am_state *state)
 {
 	const char *argv_gc_auto[] = {"gc", "--auto", NULL};
+	struct strbuf sb = STRBUF_INIT;
 
 	refresh_and_write_cache();
 
+	if (index_has_changes(&sb))
+		die(_("Dirty index: cannot apply patches (dirty: %s)"), sb.buf);
+
+	strbuf_release(&sb);
+
 	while (state->cur <= state->last) {
 		const char *mail = am_path(state, msgnum(state));