log: slowpath: do not read the full changelog

[Matt Mackall]

You pushed this patch to crew:
        
        log: slowpath: do not read the full changelog
        
        When in the slowpath, we are examining _all_ changesets in revs.
        We need to order reads so they happen increasingly for I/O performance.
        
        Increasing windows were used to read changelog backwards in a windowed manner,
        reading the changelog forward inside each window. But since no revision range
        was specified, it was equivalent to reading the full changelog, even if a
        single revision was passed to the commandline.
        
        When --removed is used, we _need_ to scan all changesets, but if we're only
        looking for file patterns, this is not necessary and we can stick to
        the revspec that was given to us.
        
And the patch contains the following:

+        if opts.get('removed'):
+            # --removed wants to yield the changes where the file
+            # was removed, this means that we have to explore all
+            # changesets, effectively ignoring the revisions that
+            # had been passed as arguments
+            revrange = xrange(nullrev, len(repo) - 1)

Not sure about this: if I say 'hg log --removed -r 1.5:1.6 hg', then I
want to know about anywhere between 1.5 and 1.6 where hg was modified,
including removals. It's not necessary to look at anything outside of
1.5:1.6. If we do look at changesets outside of that range, it's wasting
time, and if we report them, it's a bug. But it does mean that I must
look at every changelog entry in that range and not simply the filelog.

Now if I use a revspec and say:

hg log --removed -r '1.5::1.6' hg

...again, the same rules apply. I can't look at just the hg filelog, but
I needn't (shouldn't!) look at anything outside of 1.5::1.6.

And lastly, if I say:

hg log --removed -r '1.5::1.6 and file(hg)'

..I've already done all the work of finding the relevant revisions
before getting here. (Most of this code is obsoleted by revsets.)

-- 
Mathematics is the supreme nostalgia of our time.