manifest: skip fastdelta if the change is large...
manifest: skip fastdelta if the change is large
In large repos, the existing manifest fastdelta computation (which performs a
bisect on the raw manifest for every file that is changing), is excessively
slow. This patch makes fastdelta fallback to the normal string delta algorithm
if the number of changes is large.
On a large repo with a commit of 8000 files, this reduces the commit time by 7
seconds (fastdelta goes from 8 seconds to 1).
I tested this change by modifying the function to compare the old and the new
values and running the test suite. The only difference is that the pure
text-diff algorithm sometimes produces smaller (but functionaly identical)
deltatexts than the bisect algorithm.
