largefiles: optimize update speed by only updating changed largefiles Historically, during 'hg update', every largefile in the working copy was hashed (which is a very expensive operation on big files) and any largefiles that did not have a hash that matched their standin were updated. This patch optimizes 'hg update' by keeping track of what standins have changed between the old and new revisions, and only updating the largefiles that have changed. This saves a lot of time by avoiding the unecessary calculation of a list of sha1 hashes for big files. With this patch, the time 'hg update' takes to complete is a function of how many largefiles need to be updated and what their size is. Performance tests on a repository with about 80 largefiles ranging from a few MB to about 97 MB are shown below. The tests show how long it takes to run 'hg update' with no changes actually being updated. Mercurial 2.1 release: $ time hg update 0 files updated, 0 files merged, 0 files removed, 0 files unresolved getting changed largefiles 0 largefiles updated, 0 removed real 0m10.045s user 0m9.367s sys 0m0.674s With this patch: $ time hg update 0 files updated, 0 files merged, 0 files removed, 0 files unresolved real 0m0.965s user 0m0.845s sys 0m0.115s The same repsoitory, without the largefiles extension enabled: $ time hg update 0 files updated, 0 files merged, 0 files removed, 0 files unresolved real 0m0.799s user 0m0.684s sys 0m0.111s So before the patch, 'hg update' with no changes was approximately 9.25s slower with largefiles enabled. With this patch, it is approximately 0.165s slower.