git-rev-list: add --exclude-first-parent-only flag

It is useful to know when a branch first diverged in history
from some integration branch in order to be able to enumerate
the user's local changes. However, these local changes can
include arbitrary merges, so it is necessary to ignore this
merge structure when finding the divergence point.

In order to do this, teach the "rev-list" family to accept
"--exclude-first-parent-only", which restricts the traversal
of excluded commits to only follow first parent links.

   -A-----E-F-G--main
     \   / /
      B-C-D--topic

In this example, the goal is to return the set {B, C, D} which
represents a topic branch that has been merged into main branch.
`git rev-list topic ^main` will end up returning no commits
since excluding main will end up traversing the commits on topic
as well. `git rev-list --exclude-first-parent-only topic ^main`
however will return {B, C, D} as desired.

Add docs for the new flag, and clarify the doc for --first-parent
to indicate that it applies to traversing the set of included
commits only.

Signed-off-by: Jerry Zhang <jerry@skydio.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

1 files changed, 12 insertions, 6 deletions

diff --git a/t/t6012-rev-list-simplify.sh b/t/t6012-rev-list-simplify.sh
index 4f7fa8b6c0..e2851fd75d 100755
--- a/t/t6012-rev-list-simplify.sh
+++ b/t/t6012-rev-list-simplify.sh
@@ -16,13 +16,12 @@ unnote () {
 }
 
 #
-# Create a test repo with interesting commit graph:
+# Create a test repo with an interesting commit graph:
 #
-# A--B----------G--H--I--K--L
-#  \  \           /     /
-#   \  \         /     /
-#    C------E---F     J
-#        \_/
+# A-----B-----G--H--I--K--L
+#  \     \      /     /
+#   \     \    /     /
+#    C--D--E--F     J
 #
 # The commits are laid out from left-to-right starting with
 # the root commit A and terminating at the tip commit L.
@@ -142,6 +141,13 @@ check_result 'I B A' --author-date-order -- file
 check_result 'H' --first-parent -- another-file
 check_result 'H' --first-parent --topo-order -- another-file
 
+check_result 'L K I H G B A' --first-parent L
+check_result 'F E D C' --exclude-first-parent-only F ^L
+check_result '' F ^L
+check_result 'L K I H G J' L ^F
+check_result 'L K I H G B J' --exclude-first-parent-only L ^F
+check_result 'L K I H G B' --exclude-first-parent-only --first-parent L ^F
+
 check_result 'E C B A' --full-history E -- lost
 test_expect_success 'full history simplification without parent' '
 	printf "%s\n" E C B A >expect &&