Change Log:
* Fix includes from relative directories
* Add manually buffered file I/O
* Set default directory walk strategy to POSIX nftw
* Pull DFS in serial_dependency_calculation::build_indirect_includes_map out
  so it shows up properly on profiling

By replacing the sliding window with a set of fixed-width blocks,
the total number of indirect includes for any given file is given an
upper bound.
The number of indirect dependencies per file should be independent of
the number of files generated after passing a given size threshold,
currently about 1000 files.

Change Log:
* Use blocks instead of sliding windows for dependency generation
  in rattlehead.
* Add 8 MiB file I/O buffer in serial_dependency_calculation::write_includes.

In rattlehead::random_project_dependency_graph, Rattlehead uses
std::sample to generate a list of random, non-overlapping dependencies.
This has a complexity linear in its input range.
By limiting the input range, the complexity of std::sample is bounded.
By adding a limit to the input range, this effectively adds a sliding
window on the files considered for dependency by Rattlehead.

With this fix, Rattlehead generates 1 million files in 5 minutes on
a Macbook Pro.

Randomizing the centre of the sliding window could work to further
eliminate the apparent effect of a sliding window on the output.

Doing something smarter than using std::sample to generate a
non-repeating list of numbers in a bounded range would be even better.

Add test which checks megadep::megadep correctly reads and processes the
includes from a Rattlehead-generated project.

Change Log:
* Add explicit write_direct_includes and write_indirect_includes methods
  for MegadepPolicy in megadep::megadep.
* Add rm_rf function in tests/src/utilities/rm_rf.hpp, which implements
  rm -rf. Replace manual file management in tests with rm_rf.
* Move some console output from megadep::megadep to megadep.main.cpp.
* Add megadep::librattlehead alias library

The Rattlehead project is intended to be used to test the correctness of
Megadep, and to benchmark it in a fine-tunable manner.

Rattlehead generates test project layouts with nested directories and
acyclic dependencies between files.
Project files contain #include directives to a number of other files in the
project to create dependencies between files, suitable for consumption by
Megadep.

Rattlehead has been added in tests/src.

Change Log:
* Add .gitlab-ci.yml and CI scripts under tests/ci.
* Add CI tests for lint.sh, format.sh, and unit tests
* Fix missing headers
* Drop CMake version to 3.13 for Ubuntu 19.04

git-subtree-dir: external/cli11
git-subtree-split: 13becaddb657eacd090537719a669d66d393b8b2

filesystem.cpp: Use reused path buffer for directory recursion. Use std::string_view for input directory name type, and std::string for returned filename type. Remove common_types.hpp.

git-subtree-dir: external/catch2
git-subtree-split: fbf03cd31c6a1c211fa85bd620defbdc4c2e37d0