branchmap: build the revbranchcache._namesreverse() only when required On big repositories with a lot of named branches and that also increasing over time, building of this dict can be expensive and shows up in profile. For our internal repository, this saves ~0.05 seconds. Thanks to Yuya for suggesting using util.propertycache() and util.clearcachedproperty(). Differential Revision: https://phab.mercurial-scm.org/D5291

sparse-revlog: put the native implementation of slicechunktodensity to use When possible, the C implementation of delta chain slicing will be used. providing a large boost in performance for this operation. To take a practical example of restoring manifest revision '59547c40bc4c' for a reference NetBeans repository (using sparse-revlog). The media time of the step `slice-sparse-chain` of `perfrevlogrevision` improve from 0.660 ms to 0.098 ms; The full series move delta chain slicing from 1.120 ms to 0.098 ms; Implementing _slicechunktosize into C would yield further improvements. However, the performance seems good enough for now.

cext: increment the `parsers` module version We introduced a new `slicechunktodensity` method to the index.

sparse-revlog: introduce native (C) implementation of slicechunktodensity This is a C implementation of `_slicechunktodensity` in the `mercurial/revlogutils/deltas.py` file. The algorithm involves a lot of integer manipulation and low-level access to index data. Having a C implementation of it raises a large performance improvement. See later changeset in this series for details.

sparse-revlog: add a `trim_endidx` function in C We are about to implement a native version of `slicechunktodensity`. For clarity, we introduce the helper functions first. This function implement a subpart of the python function `_trimchunk` in `mercurial/revlogutils/deltas.py`. Handling of actual Python objects is left to the caller function.

sparse-revlog: add a `index_segment_span` function in C We are about to implement a native version of `slicechunktodensity`. For clarity, we introduce the helper functions first. This new function provides an efficient way to retrieve some of the information needed by `slicechunktodensity`.

sparse-revlog: add a `index_get_length` function in C We are about to implement a native version of `slicechunktodensity`. For clarity, we introduce the helper functions first. This new function provides an efficient way to retrieve some of the information needed by `slicechunktodensity`.

sparse-revlog: add a `index_get_start` function in C We are about to implement a native version of `slicechunktodensity`. For clarity, we introduce the helper functions first. This new function provides an efficient way to retrieve some of the information needed by `slicechunktodensity`.

perf: add --clear-revlog flag to branchmapload Having the changelog index already loaded when loading the branchmap can have a large impact on performance. Example runs (large private repository): hg perfbranchmapload -f base ! wall 0.116722 comb 0.120000 user 0.110000 sys 0.010000 (best of 59) hg perfbranchmapload -f base --clear-revlogs ! wall 0.258246 comb 0.230000 user 0.220000 sys 0.010000 (best of 31)

perf: introduce a function to fully "unload" a changelog The function remove various attributes and caches related to changelog. This is getting a common requirement.