merge: handle FETCH_HEAD internally

The collect_parents() function now is responsible for

 1. parsing the commits given on the command line into a list of
    commits to be merged;

 2. filtering these parents into independent ones; and

 3. optionally calling fmt_merge_msg() via prepare_merge_message()
    to prepare an auto-generated merge log message, using fake
    contents that FETCH_HEAD would have had if these commits were
    fetched from the current repository with "git pull . $args..."

Make "git merge FETCH_HEAD" to be the same as the traditional

    git merge "$(git fmt-merge-msg <.git/FETCH_HEAD)" $commits

invocation of the command in "git pull", where $commits are the ones
that appear in FETCH_HEAD that are not marked as not-for-merge, by
making it do a bit more, specifically:

 - noticing "FETCH_HEAD" is the only "commit" on the command line
   and picking the commits that are not marked as not-for-merge as
   the list of commits to be merged (substitute for step #1 above);

 - letting the resulting list fed to step #2 above;

 - doing the step #3 above, using the contents of the FETCH_HEAD
   instead of fake contents crafted from the list of commits parsed
   in the step #1 above.

Note that this changes the semantics.  "git merge FETCH_HEAD" has
always behaved as if the first commit in the FETCH_HEAD file were
directly specified on the command line, creating a two-way merge
whose auto-generated merge log said "merge commit xyz".  With this
change, if the previous fetch was to grab multiple branches (e.g.
"git fetch $there topic-a topic-b"), the new world order is to
create an octopus, behaving as if "git pull $there topic-a topic-b"
were run.  This is a deliberate change to make that happen, and
can be seen in the changes to t3033 tests.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

1 files changed, 4 insertions, 0 deletions

diff --git a/Documentation/git-merge.txt b/Documentation/git-merge.txt
index cf2c374b71..d9aa6b6f11 100644
--- a/Documentation/git-merge.txt
+++ b/Documentation/git-merge.txt
@@ -104,6 +104,10 @@ commit or stash your changes before running 'git merge'.
 If no commit is given from the command line, merge the remote-tracking
 branches that the current branch is configured to use as its upstream.
 See also the configuration section of this manual page.
++
+When `FETCH_HEAD` (and no other commit) is specified, the branches
+recorded in the `.git/FETCH_HEAD` file by the previous invocation
+of `git fetch` for merging are merged to the current branch.
 
 
 PRE-MERGE CHECKS