Turn off log.decorate when parsing log output

[avar]

Given lines like:

commit 0245492ee0
commit 5b0af9d6f8 (sahildua/add-copy-branch-feature)

This code would error out with:

Traceback (most recent call last):
  File "/home/avar/bin/git-tbdiff", line 400, in <module>
    sA, dA = read_patches(rangeA)
  File "/home/avar/bin/git-tbdiff", line 92, in read_patches
    _, sha1 = line.strip().split()
ValueError: too many values to unpack

Fix that by turning off decorate for the log invocation, and while I'm
at it turn off commit abbreviation which isn't needed here either.

[avar]

I see after opening this that this is a dupe of #7, but the description here is more detailed & this fixes another bug, so it makes sense to merge this one & close the other one unmerged.

[dscho]

while I'm at it turn off commit abbreviation which isn't needed here either.

I think this merits a bit more explanation. How about this?

The abbreviation is *not* used for the `commit <commit-name>` lines
anyway, but only for the `index` lines, which are *ignored* by tbdiff.
And as abbreviation does not necessarily come cheap (see e.g.
https://github.com/gitster/git/commits/ds/find-unique-abbrev-optim), 
let's just avoid the unnecessary churn.

[tsibley]

I've merged this PR into my fork: https://github.com/tsibley/git-tbdiff

Thanks for the patches!