remote: store the connection data in a private struct

This makes it easier to pass connection-related options around (proxy &
custom headers for now).

This fixes a bug in git_push_finish, which didn't reuse the provided
proxy if the connection closed between the call to `git_remote_push` and
the finish step.

In case where we add multiple configuration entries with the same key to
a diskfile backend, we always need to iterate the list of this key to
find the last entry due to the list being a singly-linked list. This
is obviously quadratic behaviour, and this has sure enough been found by
oss-fuzz by generating a configuration file with 50k lines, where most
of them have the same key. While the issue will not arise with "sane"
configuration files, an adversary may trigger it by providing a crafted
".gitmodules" file, which is delivered as part of the repo and also
parsed by the configuration parser.

The fix is trivial: store a pointer to the last entry of the list in its
head. As there are only two locations now where we append to this data
structure, mainting this pointer is trivial, too. We can also optimize
retrieval of a single value via `config_get`, where we previously had to
chase the `next` pointer to find the last entry that was added.

Using our configuration file fozzur with a corpus that has a single file
with 50000 "-=" lines previously took around 21s. With this optimization
the same file scans in about 0.053s, which is a nearly 400-fold
improvement. But in most cases with a "normal" amount of same-named keys
it's not going to matter anyway.

config: Fix a leak parsing multi-line config entries

When running repack while doing repo writes, `packfile_load__cb()` can see some temporary files in the directory that are bigger than the usual, and makes `memcmp` overflow on the `p->pack_name` string. ASAN detected this. This just uses `strncmp`, that should not have any performance impact and is safe for comparing strings of different sizes.

```
ERROR: AddressSanitizer: heap-buffer-overflow on address 0x61200001a3f3 at pc 0x7f4a9e1976ec bp 0x7ffc1f80e100 sp 0x7ffc1f80d8b0
READ of size 89 at 0x61200001a3f3 thread T0
SCARINESS: 26 (multi-byte-read-heap-buffer-overflow)
    #0 0x7f4a9e1976eb in __interceptor_memcmp.part.78 (/build/cfgr-admin#link-tree/libtools_build_sanitizers_asan-ubsan-py.so+0xcf6eb)
    #1 0x7f4a518c5431 in packfile_load__cb /build/libgit2/0.27.0/src/libgit2-0.27.0/src/odb_pack.c:213
    #2 0x7f4a518d9582 in git_path_direach /build/libgit2/0.27.0/src/libgit2-0.27.0/src/path.c:1134
    #3 0x7f4a518c58ad in pack_backend__refresh /build/libgit2/0.27.0/src/libgit2-0.27.0/src/odb_pack.c:347
    #4 0x7f4a518c1b12 in git_odb_refresh /build/libgit2/0.27.0/src/libgit2-0.27.0/src/odb.c:1511
    #5 0x7f4a518bff5f in git_odb__freshen /build/libgit2/0.27.0/src/libgit2-0.27.0/src/odb.c:752
    #6 0x7f4a518c17d4 in git_odb_stream_finalize_write /build/libgit2/0.27.0/src/libgit2-0.27.0/src/odb.c:1415
    #7 0x7f4a51b9d015 in Repository_write /build/pygit2/0.27.0/src/pygit2-0.27.0/src/repository.c:509
```

The current error handling for the multiline variable parser is a bit
fragile, as each error condition has its own code to clear memory.
Instead, unify error handling as far as possible to avoid this
repetitive code. While at it, make use of `GITERR_CHECK_ALLOC` to
correctly handle OOM situations and verify that the buffer we print into
does not run out of memory either.

config: convert unbounded recursion into a loop

util: make the qsort_r check work on macOS

tree: accept null ids in existing trees when updating