Re: git pull --set-upstream segfaults on branchless repo.

> What did you do before the bug happened? (Steps to reproduce your
> issue)
> 
> Run "git pull origin nixos-unstable --rebase --set-upstream"
> on a repo that had no branch set, e.g. when running "git branch" gave
> "* (no branch)"
> 
> What did you expect to happen? (Expected behavior)
> Pull from upstream and set upstream.
> 
> What happened instead? (Actual behavior)
> Segfault with the following trace:
> (gdb) bt
> #0  0x000000000044a8c9 in do_fetch (transport=0x2557920, rs=rs@entry=0x7ffd42335c00) at builtin/fetch.c:1568
> #1  0x000000000044ac61 in fetch_one (remote=<optimized out>, argc=1110662144, argv=0x7ffd42335fc8, prune_tags_ok=<optimized out>, use_stdin_refspecs=0) at builtin/fetch.c:1892
> #2  0x000000000044af15 in cmd_fetch (argc=0, argv=0x7ffd42335fc8, prefix=0x0) at builtin/fetch.c:1992
> #3  0x0000000000406354 in run_builtin (p=0x70d3e0 <commands+960>, argc=argc@entry=5, argv=argv@entry=0x7ffd42335fc0) at git.c:453
> #4  0x00000000004065c3 in handle_builtin (argc=5, argv=0x7ffd42335fc0) at git.c:704
> #5  0x0000000000407c4a in run_argv (argcp=argcp@entry=0x7ffd42335e8c, argv=argv@entry=0x7ffd42335e80) at git.c:771
> #6  0x00000000004080a4 in cmd_main (argc=<optimized out>, argc@entry=6, argv=<optimized out>, argv@entry=0x7ffd42335fb8) at git.c:902
> #7  0x00000000004c614c in main (argc=6, argv=0x7ffd42335fb8) at common-main.c:52
> (gdb) p branch
> $1 = (struct branch *) 0x0
> 
> What's different between what you expected and what actually happened?
> Not segfault
> 
> Anything else you want to add:
> 
> Dropping the "--set-upstream" makes the segfault go away. Looking at
> builtin/fetch.c around L1568 I see,
> 
>         if (set_upstream) {
>                 struct branch *branch = branch_get("HEAD");
>                 [..]
>                 if (source_ref) {
>                         if (!strcmp(source_ref->name, "HEAD") ||
>                             starts_with(source_ref->name, "refs/heads/"))
>                                 install_branch_config(0,
>                                                       branch->name, // <- SEGFAULT HERE
>                                                       transport->remote->name,
>                                                       source_ref->name);
>                         [..]
>               }
>         }
> 
> It's rather clear that branch is just NULL from the gdb session above,
> and the branch->name dereference fails.
> It might be useful to catch branch == NULL.
> 
> Please review the rest of the bug report below.
> You can delete any lines you don't wish to share.
> 
> [System Info]
> git version:
> git version 2.31.1
> [...]