[PATCH 3 of 3] revlog: optionally cache the full text when adding revisions

[timeless]

> I was doing some performance analysis on my Windows 10 machine (no VM,
SSD, NTFS) and this patch has a drastic impact.

> Applying just the first 100,000 changesets of a gzip bundle of
mozilla-central went from 90-110s to 16.5-17.0s. That's a 5x speedup!

> My takeaway is [re?]opening files [for writing?] in rapid succession
isn't very efficient on Windows. I know people like to knock I/O on
Windows. But I didn't think it was this bad. Part of me suspects we
(possibly Python) is doing something really sub-optimal.

Typically this is the antivirus hook. In a default configuration, that's
Windows Defender. (I haven't checked to see if I'm right, I'm traveling.)

You can use ProcMon from sys internals to log accesses to a directory.
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