Previously, we call differential.getrawdiff per patch despite we have the
diff content already (returned by differential.querydiffs).

This patch implements the serialization logic of differential.getrawdiff
client-side so we no longer need calling differential.getrawdiff. This
removes one API call per patch.

Now fetching a stack of 10 patches is just 2 API calls in the best case.
Worst case batching does not work and it's 11 API calls. The "reading"
verbose message was removed since there is no remote IO needed to read a
patch in that loop now.

Using a test Phabricator instance, this patch reduces phabread reading a
10-patch stack from about 8 seconds to 1.3 seconds. Even with batching
disabled, reading that 10-patch stack could be done in 4 seconds. The
copy-paste constants are a bit unfortunate since "code is documentation" is
true for many Phabricator features. But the perf win could probably justify
it.

Create an interesting (contains multiple copies and renames, mixed mode
change with content changes) patch using the below script:

hg init

# long context could be more interesting
seq 1 200 > long
for i in a b c d e f g; do
  echo $i > $i
done
hg add .
hg commit -m init

sed -i '/10/d;s/22$/22.1\n22.2/g' long

# multiple copies and moves
for i in a b; do
  for j in 1 2 3 4; do
    hg cp $i "${i}${j}"
    [ $((j % 2)) = 0 ] && chmod +x "${i}${j}"
    [ $j -ge 3 ] && echo append >> "${i}${j}"
  done
done

hg rm a
chmod +x c
hg rm d
hg mv e E
chmod +x E
rm f
touch f
hg commit -m change

Send it using phabsend, and retrieve it using phabread. Compare before
and after this patch. The only differences are:

• This patch removes a blank line after a patch.
• This patch does not omit ,1 in diff hunks. See [1].

Other than that, the patch output is exactly the same. So the new code path
looks reasonably good.

Also try using phabread to read all existing diffs sent to the website and
make sure it works.

[1]: https://secure.phabricator.com/diffusion/ARC/browse/master/src/parser/
ArcanistBundle.php;e00e2939c164df85852ce2d157de4197b399c30b$631