Visual Studio Team Services is now a family of applications named "Azure
DevOps".  Update the README to refer to it thusly.

VSTS is now a family of components; "Azure Pipelines" is the build and
release pipeline application.

Fix revwalk limiting regression

path validation: `char` is not signed by default.

revwalk: refer the sorting modes more to git's options

Show more directly what the sorting modes correspond to in git's `rev-list` as
that's the reference implementation for what the possible sorting orders are.

This is not a big deal, but it does make us match git more closely by checking
only the first. The lists are sorted already, so there should be no functional
difference other than removing a possible check from every iteration in the
loop.

When porting, we overlooked that the difference between git's and our's time
representation and copied their way of getting the max value.

Unfortunately git was using unsigned integers, so `~0ll` does correspond to
their max value, whereas for us it corresponds to `-1`. This means that we
always consider the last date to be smaller than the current commit's and always
think commits are interesting.

Change the initial value to the macro that gives us the maximum value on each
platform so we can accurately consider commits interesting or not.

ARM treats its `char` type as `unsigned type` by default; as a result,
testing a `char` value as being `< 0` is always false.  This is a
warning on ARM, which is promoted to an error given our use of
`-Werror`.

Per ISO 9899:199, section "6.2.5 Types":

> The three types char, signed char, and unsigned char are collectively
> called the character types. The implementation shall define char to
> have the same range, representation, and behavior as either signed
> char or unsigned char.
>
...

> Irrespective of the choice made, char is a separate type from the other
> two and is not compatible with either.