[PATCH 0/3] show alias definitions in help

This was requested during the freeze period, and I posted a "like this?"
patch with a promise to follow-up post-1.5.4. So here is the follow-up.

The objections to my quick patch were:

  - (Dscho) the parsing code was ugly and needed cleanup; the first
    patch refactors cmd_help to use parseopt
  - (Duy) help lookup should respect the usual "commands before alias"
    rule that we use for execution; the third patch takes this into
    account

      1/3: help: use parseopt
           This is parseopt plus general cleanup.

      2/3: make alias lookup a public, procedural function
           Another cleanup to factor out alias lookup from git.c (which
           we will also need in help now).

      help: respect aliases
           The actual patch.

 Makefile |    3 +-
 alias.c  |   22 +++++++++
 cache.h  |    2 +
 git.c    |   17 +------
 help.c   |  152 ++++++++++++++++++++++++++++++++++++--------------------------
 5 files changed, 118 insertions(+), 78 deletions(-)
 create mode 100644 alias.c