merge: add mergeresult.mapaction to improve speed As a part of [hg update] we convert all [ACTION_CREATED] merge results into [ACTION_GET] actions, and that's slightly inefficient because every insertion pays the full cost of maintaining the [mergeresult] data structure up to date. This commit adds a function [mapaction], which is faster. (saves around 0.3s on a large update involving ~400k files)

merge: avoid dereferencing repo fields repeatedly

merge: skip syntactic path checks in [_checkunknownfile] We don't need to check the paths syntactically, since they are coming from diffing the revisions, so hopefully already checked on the way in. We still need to check what's on the filesystem, to avoid traversing the symlinks or subdirs, which we can't know about statically. Also, we use the directory audit to elide [isfileorlink], this removing ~all lstat calls from hg updates from-empty.

pathutil: use `finddirs_rev_noroot` instead of `parts` The benefit this brings is very tiny, if it's even there, since we still didn't get rid of the [parts] computation. It probably won't be worth it without the subsequent patch that adds one more use of [finddirs_rev_noroot]

pathutil: add the more efficient finddir iterator (to be used in subsequent commits)

typing: import unconditionally This is needed if we're writing python3 signatures.

tests: check how hgweb handles HEAD requests This test file is loosely based on test-hgweb.t. HEAD support originally implemented in fda5a4b853ab.