builtin-rm.c: explain and clarify the "local change" logic

Explain the logic to check local modification a bit more in the comment,
especially because the existing comment that talks about "git rm --cached"
was placed in a part that was not about "--cached" at all.

Also clarify "if .. else if .." structure.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin-rm.c

@@ -31,7 +31,8 @@ static void add_list(const char *name)
 
 static int check_local_mod(unsigned char *head, int index_only)
 {
-	/* items in list are already sorted in the cache order,
+	/*
+	 * Items in list are already sorted in the cache order,
 	 * so we could do this a lot more efficiently by using
 	 * tree_desc based traversal if we wanted to, but I am
 	 * lazy, and who cares if removal of files is a tad
@@ -71,25 +72,55 @@ static int check_local_mod(unsigned char *head, int index_only)
 			 */
 			continue;
 		}
+
+		/*
+		 * "rm" of a path that has changes need to be treated
+		 * carefully not to allow losing local changes
+		 * accidentally.  A local change could be (1) file in
+		 * work tree is different since the index; and/or (2)
+		 * the user staged a content that is different from
+		 * the current commit in the index.
+		 *
+		 * In such a case, you would need to --force the
+		 * removal.  However, "rm --cached" (remove only from
+		 * the index) is safe if the index matches the file in
+		 * the work tree or the HEAD commit, as it means that
+		 * the content being removed is available elsewhere.
+		 */
+
+		/*
+		 * Is the index different from the file in the work tree?
+		 */
 		if (ce_match_stat(ce, &st, 0))
 			local_changes = 1;
+
+		/*
+		 * Is the index different from the HEAD commit?  By
+		 * definition, before the very initial commit,
+		 * anything staged in the index is treated by the same
+		 * way as changed from the HEAD.
+		 */
 		if (no_head
 		     || get_tree_entry(head, name, sha1, &mode)
 		     || ce->ce_mode != create_ce_mode(mode)
 		     || hashcmp(ce->sha1, sha1))
 			staged_changes = 1;
 
-		if (local_changes && staged_changes &&
-		    !(index_only && is_empty_blob_sha1(ce->sha1)))
-			errs = error("'%s' has staged content different "
-				     "from both the file and the HEAD\n"
-				     "(use -f to force removal)", name);
+		/*
+		 * If the index does not match the file in the work
+		 * tree and if it does not match the HEAD commit
+		 * either, (1) "git rm" without --cached definitely
+		 * will lose information; (2) "git rm --cached" will
+		 * lose information unless it is about removing an
+		 * "intent to add" entry.
+		 */
+		if (local_changes && staged_changes) {
+			if (!index_only || !is_empty_blob_sha1(ce->sha1))
+				errs = error("'%s' has staged content different "
+					     "from both the file and the HEAD\n"
+					     "(use -f to force removal)", name);
+		}
 		else if (!index_only) {
-			/* It's not dangerous to "git rm --cached" a
-			 * file if the index matches the file or the
-			 * HEAD, since it means the deleted content is
-			 * still available somewhere.
-			 */
 			if (staged_changes)
 				errs = error("'%s' has changes staged in the index\n"
 					     "(use --cached to keep the file, "