merge: allow to use only the fp-only merge bases

Teach "git merge" a new option "--fp-base-only" that tells it to
consider only merge bases that are on the first-parent chain.

This speeds up back-merges needed in the topic branch workflow.  The
merge of 'master' back into 'next' used as an example in the RFD
article <[email protected]>, i.e.

    git checkout 4868def && git merge 6598894

in our history takes about 1.22-1.33 seconds without the series,
while running the latter "git merge" with the "--fp-base-only"
option takes about 0.54-0.59 seconds.

Signed-off-by: Junio C Hamano <[email protected]>

Author: gitster

Documentation/merge-options.txt

@@ -116,6 +116,15 @@ ifndef::git-pull[]
 	Note that not all merge strategies may support progress
 	reporting.
 
+--fp-base-only::
+	Instead of using all merge bases when computing the
+	three-way merge result, use only the merge bases on the
+	first-parent chain of the commits being merged.  This
+	experimental feature is meant to be used when merging an
+	older integration branch back to a newer integration branch
+	in the topic-branch workflow.
+
+
 endif::git-pull[]
 
 --allow-unrelated-histories::

builtin/merge.c

@@ -88,6 +88,8 @@ enum ff_type {
 
 static enum ff_type fast_forward = FF_ALLOW;
 
+static int fp_base_only;
+
 static int option_parse_message(const struct option *opt,
 				const char *arg, int unset)
 {
@@ -210,6 +212,8 @@ static struct option builtin_merge_options[] = {
 	{ OPTION_SET_INT, 0, "ff-only", &fast_forward, NULL,
 		N_("abort if fast-forward is not possible"),
 		PARSE_OPT_NOARG | PARSE_OPT_NONEG, NULL, FF_ONLY },
+	OPT_BOOL(0, "fp-base-only", &fp_base_only,
+		 N_("use only merge bases on first-parent chain")),
 	OPT_RERERE_AUTOUPDATE(&allow_rerere_auto),
 	OPT_BOOL(0, "verify-signatures", &verify_signatures,
 		N_("verify that the named commit has a valid GPG signature")),
@@ -1340,9 +1344,14 @@ int cmd_merge(int argc, const char **argv, const char *prefix)
 
 	if (!remoteheads)
 		; /* already up-to-date */
-	else if (!remoteheads->next)
-		common = get_merge_bases(head_commit, remoteheads->item);
-	else {
+	else if (!remoteheads->next) {
+		unsigned flags = MB_POSTCLEAN;
+		if (fp_base_only)
+			flags |= MB_FPCHAIN;
+		common = get_merge_bases_opt(head_commit,
+					     1, &remoteheads->item,
+					     flags);
+	} else {
 		struct commit_list *list = remoteheads;
 		commit_list_insert(head_commit, &list);
 		common = get_octopus_merge_bases(list);