shallow.c: implement a generic shallow boundary finder based on rev-list

Instead of a custom commit walker like get_shallow_commits(), this new
function uses rev-list to mark NOT_SHALLOW to all reachable commits,
except borders. The definition of reachable is to be defined by the
protocol later. This makes it more flexible to define shallow boundary.

The way we find border is paint all reachable commits NOT_SHALLOW.  Any
of them that "touches" commits without NOT_SHALLOW flag are considered
shallow (e.g. zero parents via grafting mechanism). Shallow commits and
their true parents are all marked SHALLOW. Then NOT_SHALLOW is removed
from shallow commits at the end.

There is an interesting observation. With a generic walker, we can
produce all kinds of shallow cutting. In the following graph, every
commit but "x" is reachable. "b" is a parent of "a".

           x -- a -- o
          /    /
    x -- c -- b -- o

After this function is run, "a" and "c" are both considered shallow
commits. After grafting occurs at the client side, what we see is

                a -- o
                    /
         c -- b -- o

Notice that because of grafting, "a" has zero parents, so "b" is no
longer a parent of "a".

This is unfortunate and may be solved in two ways. The first is change
the way shallow grafting works and keep "a -- b" connection if "b"
exists and always ends at shallow commits (iow, no loose ends). This is
hard to detect, or at least not cheap to do.

The second way is mark one "x" as shallow commit instead of "a" and
produce this graph at client side:

           x -- a -- o
               /    /
         c -- b -- o

More commits, but simpler grafting rules.

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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