FileStructureWalker (pass HgFile, HgFolder to callable; which can ask for VCS data from any file)
External uses: user browses files, selects one and asks for its history 
Params: tip/revision; 
Implementation: manifest

Log --rev
Log <file>
HgDataFile.history() or Changelog.history(file)?


Changelog.all() to return list with placeholder, not-parsed elements (i.e. read only compressedLen field and skip to next record), so that
total number of elements in the list is correct

hg cat
Implementation: logic to find file by name in the repository is the same with Log and other commands


Revlog
What happens when big entry is added to a file - when it detects it can't longer fit into .i and needs .d? Inline flag and .i format changes?


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support patch from baseRev + few deltas
+ command-line samples (-R, filenames) (Log & Cat) to show on any repo

input stream impl + lifecycle
delta merge