[PATCH 0/7] Allow custom merge strategies

Hi,

This series adds support for custom merge strategies.

The first 3 patches modify builtin-help to allow using it from other
builtins. This is necessary because in the error message of 'git merge
-s foobar' we show something like 'git help -a', but we list only merge
strategies. A command is considered a merge strategy if it has a
git-merge- prefix and is listed in the all_strategy array or it is
somewhere in PATH, but outside `git --exec-path`, so that git-merge-ours
and other strategies are shown, git-merge-index and other
git-merge-named (but not strategy) commands are hidden.

The last two is about removing those problematic 'git-merge-index',
'git-merge-tree' and other bogus commands from the output of 'git merge
-s foobar'. I think the benefit of doing it that way is that we don't
have to maintain a list of commands which are named git-merge-foo but
not strategies _and_ the custom strategies can have a form of
git-merge-foo, without adding extra complexity (like forcing users to
name them git-merge-custom-foo).

NOTE: At the moment the custom strategies are named as git-merge-foo as
well, mainly because I think it's not that problematic to exclude the
already existing git-merge-fo non-strategy commands, but this can be
changed to git-merge-strategy-foo if we really want so.

Also, I'm aware that this is a feature and we are in rc freeze, I just
did not want to keep back this series till 1.6.0 is out.

Miklos Vajna (7):
  Make is_git_command() usable outside builtin-help
  builtin-help: change the current directory back in
    list_commands_in_dir()
  builtin-help: make list_commands() a bit more generic
  builtin-merge: allow using a custom strategy
  Add a new test for using a custom merge strategy
  builtin-help: make it possible to exclude some commands in
    list_commands()
  builtin-merge: avoid non-strategy git-merge commands in error message

 Makefile                |    1 +
 builtin-merge.c         |   30 +++++++++++++++++++++------
 help.c                  |   50 ++++++++++++++++++++++++----------------------
 help.h                  |   19 +++++++++++++++++
 t/t7606-merge-custom.sh |   45 ++++++++++++++++++++++++++++++++++++++++++
 5 files changed, 114 insertions(+), 31 deletions(-)
 create mode 100644 help.h
 create mode 100755 t/t7606-merge-custom.sh