Dont count on magic value "master" for the git backend

[6543]

There still exist a lot of string values in the codebase of "master"

we at least should use one single const.
but it's better to respect repo specific settings if posible

we should try to determine what the default branch of a repo or at least of the system is

Originally posted by @6543 in #23894 (comment)

[silverwind]

1. check per-repo setting
2. fall back to global default

Indeed we should have no hardcoded master strings in the code anywhere.

[wxiaoguang]

When I did Use "main" as default branch name #19354 , there are only 2 cases left:

1. The test code
2. The wiki

Otherwise, I do not see still exist a lot of string values in the codebase of "master"

[wxiaoguang]

I think it isn't a problem now?

[6543]

yes it was continiously worked on so I guess it is now fixed :)