This is more or less a rewrite of cg-log, that adds a bunch
of new features and gives a substantial speedup.

cg-log now lets git-rev-list and git-diff-tree do as much
of the heavy lifting as possible. When possible, cg-log
pretty much reduces to a sed script, that does some fixup
and adds colorization. Otherwise, cg-log uses
--pretty=[raw|medium|short] output, that's augmented as
necessary.

New features:

  - New, human friendlier output format. cg-log now uses
    the same format as git-rev-list --pretty. It also
    doesn't show tree, committer and parents by default.
    These are all shown with "-v or --verbose", however.

  - Slightly more readable summary format. sha1s are hidden
    unless -v is given. Date format is condenced to HH:MM
    for the last day, and Month Day otherwise. (+year is
    shown for really old stuff).

    summary format can also be combined with other options,
    like "-f" and "-d" (diffstat).

  - "-u" (or --search=) now searches the entire commit
    message. This is more flexible, as you can still do e.g.
    "cg-log -u 'author Petr Baudis'". Search is now also
    a lot faster, done with a single grep invocation.

  - Output can be limited by date or count.

  - New format for "-f". This is pretty much the same
    output as cg-status gives, and allows for the
    rename/copy detection things to work. New and
    deleted files are optionally color coded.

    The old format is still available as -F.

  - Merge commits are hidden by default when displaying
    differences between commits, or when displaying disjoint
    sets of commits. "-a" makes them show again. So
    "cg-log -caF" gives you pretty much the same output
    as the old "cg-log -c -f" would.

  - Shows diffs between commits with the "-p" options.
    The diffs use the same colorization rules as cg-diff,
    and can be skipped through with "n" in less if "-c"
    is given.

  - "--stdin" reads sha1s from stdin instead of from
    git-rev-list, for some extra flexibility.

New features in common with cg-diff:

  - diffstat support (or rather git-apply --stat). The
    "-d" option outputs (optionally colorized) diff stats
    before the diff.

  - support for more git-diff-[tree/cache] options:
    -B, -R, -M, -C are now all passed on.

  - The COGITO_AUTO_COLOR environment variable makes output
    automatically colorized, if set and if we're on a color
    capable terminal.

We also use the new optparse function from cg-Xlib, to allow
for e.g. "cg-log -sfa" and stuff.

Reuse colorization logic from cg-Xlib. All the colors are
added in a single sed invocation.

And use LESS to make "less" search for chunks and diff --git
markers. This allows you to "n" your way through a series
of diffs, and gives a nice visual separation of patches.

Signed-off-by: Dan Holmsand <holmsand@gmail.com>
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