Windows lacks %F and %T formats for strftime.  Expand them to the
year/month/day and hour/minute/second formats, respectively.

Our build YAML is becoming unweildly and full of copy-pasta.  Simplify
with templates.

Explicitly run from the build directory, not the source.  (I was
mistaken about the default working directory for VSTS agents.)

CMake treats backslashes as escape characters; use forward slashes for
the XML output path.

Remove the global summary filename and file pointer; pass them in to the
summary functions as needed.  Error check the results of buffered I/O
calls.

Introduce a CLAR_XML option, to run the `ctest` commands with the new
`-r` flag to clar.  Permitted values are `OFF`, `ON` and a directory to
write the XML test results to.

Accept an (optional) value for the summary filename.  Continues to
default to summary.xml.

This makes it possible to keep track of every test status (even
successful ones), and their errors, if any.

Previously, supplying `-s` to explicitly enable some test(s) would run
the tests immediately from the argument parser.  This forces us to set
up the entire clar environment (for example: sandboxing) before argument
parsing takes place.

Refactor the behavior of `-s` to add the explicitly chosen tests to a
list that is executed later.  This untangles the argument parsing from
the setup lifecycle, allowing us to use the arguments to perform the
setup.

config: Fix a leak parsing multi-line config entries

Prevent heap-buffer-overflow

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
```

ci: remove travis

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.

Update VSTS YAML files with the latest syntax

Documentation fixes

config: convert unbounded recursion into a loop

Document giterr_last() use only after error. #4772

util: make the qsort_r check work on macOS

fuzzer: update for indexer changes

tree: accept null ids in existing trees when updating

Pack file verification

This performs a compile-check by using CMake support, to differentiate the GNU
version from the BSD version of qsort_r.

Module taken from 4f252abea5f1d17c60f6ff115c9c44cc0b6f1df6, which I've checked
against CMake 2.8.11.

cmake: detect and use libc-provided iconv