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>