revlog: implement changelogv2 packing and unpacking in C This introduces a C implementation of changelogv2 records packing and unpacking operations matching the pure Python counterpart, similarly to what we already have for revlogv1 and revlogv2. This is also necessary to access changelogv2 record fields from future Rust code without going through the Python part, which would annihilate any performance benefit. Differential Revision: https://phab.mercurial-scm.org/D12178

revlog: memset whole record instead of dealing with padding This is less error-prone. Differential Revision: https://phab.mercurial-scm.org/D12177

revlog: do not compute node location by hand in index_invalidate_added The node is not guaranteed to be at the same location all the time (e.g: changelog v2), so let's use the official existing API to get that value. Differential Revision: https://phab.mercurial-scm.org/D12176

rank: compute property incrementally This replaces the naive rank computation with a more efficient incremental method, avoiding computing the whole ancestor set when possible. Differential Revision: https://phab.mercurial-scm.org/D12143

rank: add test with golden values This adds a regression test for the computation of the rank, using the current values computed with the naive algorithm as the "golden" reference. Differential Revision: https://phab.mercurial-scm.org/D12142

rank: add minimal test This adds a small test checking the rank computation in the case of a merge. Differential Revision: https://phab.mercurial-scm.org/D12141

rank: add context and template keyword This makes the stored rank property accessible, to be expanded and printed. Differential Revision: https://phab.mercurial-scm.org/D12140

rank: naive rank property computation and retrieval This stores the rank (size of the ancestor set of a revision, including itself) in a changelog field and allows this property to be retrieved. This new property is used as part of stable-range computations, which will be introduced later on. The value is computed in a naive way from the definition of the rank. This will be replaced by a more efficient version subsequently. Differential Revision: https://phab.mercurial-scm.org/D12139

filemerge: move check for identical sides out of filemerge() `filemerge.filemerge()` returns `None` if no merge was necessary because the two sides were identical. I don't think it should be that function's responsibility to handle that case; we should ideally not even call `filemerge.filemerge()` if the two inputs identical. This patch therefore moves the check out to the caller (`mergestate.py`). The largefiles test changed because we now notice that the two sides made the same change, so we don't consider it a merge. Also note that the new message better matches the line above it in the test output. Differential Revision: https://phab.mercurial-scm.org/D12154

mergestate: use an early return for trivial merges `filemerge.filemerge()` returns `None` if no merge was needed because the two sides were identical. I'd like to move that to the caller. This is a little refactoring to prepare for that. Differential Revision: https://phab.mercurial-scm.org/D12153