builtin-commit: use reduce_heads() only when appropriate

Since commit 6bb6b034 (builtin-commit: use commit_tree(), 2008-09-10),
builtin-commit performs a reduce_heads() unconditionally.  However,
it's not always needed, and in some cases even harmful.

reduce_heads() is not needed for the initial commit or for an
"ordinary" commit, because they don't have any or have only one
parent, respectively.

reduce_heads() must be avoided when 'git commit' is run after a 'git
merge --no-ff --no-commit', otherwise it will turn the
non-fast-forward merge into fast-forward.  For the same reason,
reduce_heads() must be avoided when amending such a merge commit.

To resolve this issue, 'git merge' will write info about whether
fast-forward is allowed or not to $GIT_DIR/MERGE_MODE.  Based on this
info, 'git commit' will only perform reduce_heads() when it's
committing a merge and fast-forward is enabled.

Also add test cases to ensure that non-fast-forward merges are
committed and amended properly.

Signed-off-by: Miklos Vajna <vmiklos@frugalware.org>
Signed-off-by: SZEDER Gábor <szeder@ira.uka.de>
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>

1 files changed, 1 insertions, 0 deletions

diff --git a/branch.c b/branch.c
index b1e59f2196..205b89dc69 100644
--- a/branch.c
+++ b/branch.c
@@ -168,5 +168,6 @@ void remove_branch_state(void)
 	unlink(git_path("MERGE_HEAD"));
 	unlink(git_path("MERGE_RR"));
 	unlink(git_path("MERGE_MSG"));
+	unlink(git_path("MERGE_MODE"));
 	unlink(git_path("SQUASH_MSG"));
 }