merge(3p/git): Merge git subtree at v2.29.2

This also bumps the stable nixpkgs to 20.09 as of 2020-11-21, because
there is some breakage in the git build related to the netrc
credentials helper which someone has taken care of in nixpkgs.

The stable channel is not used for anything other than git, so this
should be fine.

Change-Id: I3575a19dab09e1e9556cf8231d717de9890484fb

third_party/git/builtin/mv.c

@@ -132,6 +132,7 @@ int cmd_mv(int argc, const char **argv, const char *prefix)
 	struct stat st;
 	struct string_list src_for_dst = STRING_LIST_INIT_NODUP;
 	struct lock_file lock_file = LOCK_INIT;
+	struct cache_entry *ce;
 
 	git_config(git_default_config, NULL);
 
@@ -220,9 +221,11 @@ int cmd_mv(int argc, const char **argv, const char *prefix)
 				}
 				argc += last - first;
 			}
-		} else if (cache_name_pos(src, length) < 0)
+		} else if (!(ce = cache_file_exists(src, length, ignore_case))) {
 			bad = _("not under version control");
-		else if (lstat(dst, &st) == 0 &&
+		} else if (ce_stage(ce)) {
+			bad = _("conflicted");
+		} else if (lstat(dst, &st) == 0 &&
 			 (!ignore_case || strcasecmp(src, dst))) {
 			bad = _("destination exists");
 			if (force) {