Be more user-friendly when refusing to do something because of conflict.

Be more user-friendly when refusing to do something because of conflict.

Various commands refuse to run in the presence of conflicts (commit,
merge, pull, cherry-pick/revert). They all used to provide rough, and
inconsistant error messages.

A new variable advice.resolveconflict is introduced, and allows more
verbose messages, pointing the user to the appropriate solution.

For commit, the error message used to look like this:

$ git commit
foo.txt: needs merge
foo.txt: unmerged (c34a92682e0394bc0d6f4d4a67a8e2d32395c169)
foo.txt: unmerged (3afcd75de8de0bb5076942fcb17446be50451030)
foo.txt: unmerged (c9785d77b76dfe4fb038bf927ee518f6ae45ede4)
error: Error building trees

The "need merge" line is given by refresh_cache. We add the IN_PORCELAIN
option to make the output more consistant with the other porcelain
commands, and catch the error in return, to stop with a clean error
message. The next lines were displayed by a call to cache_tree_update(),
which is not reached anymore if we noticed the conflict.

The new output looks like:

U       foo.txt
fatal: 'commit' is not possible because you have unmerged files.
Please, fix them up in the work tree, and then use 'git add/rm <file>' as
appropriate to mark resolution and make a commit, or use 'git commit -a'.

Pull is slightly modified to abort immediately if $GIT_DIR/MERGE_HEAD
exists instead of waiting for merge to complain.

The behavior of merge and the test-case are slightly modified to reflect
the usual flow: start with conflicts, fix them, and afterwards get rid of
MERGE_HEAD, with different error messages at each stage.

Signed-off-by: Matthieu Moy <Matthieu.Moy@imag.fr>
Signed-off-by: Junio C Hamano <gitster@pobox.com>