dirstate: avoid unnecessary load+dump during backup Previously, dirstate.savebackup unconditionally dumps the dirstate map to disk. It may require loading dirstate first to be able to dump it. Those operations could be expensive if the dirstate is big, and could be avoided if we know the dirstate file is up-to-date. This patch avoids the read and write if the dirstate is clean. In that case, we just do a plain copy without any serialization. This should make commands which use transactions but do not touch dirstate faster. For example, "hg bookmark -r REV NAME".

dirstate: try to use hardlink to backup dirstate This should be more efficient once util.copyfile has real hardlink support.

dirstate: track updated files to improve write time Previously, dirstate.write() would iterate over the entire dirstate to find any entries that needed to be marked 'lookup' (i.e. if they have the same timestamp as now). This was O(working copy) and slow in large repos. It was most visible when rebasing or histediting multiple commits, since it gets executed once per commit, even if the entire rebase/histedit is wrapped in a transaction. The fix is to track which files have been editted, and only check those to see if they need to be marked as 'lookup'. This saves 25% on histedit times in very large repositories. I tested this by adding temporary debug logic to verify that the old files processed in the loop matched the new files processed in the loop and running the test suite.