builtin/history: implement "split" subcommand

It is quite a common use case that one wants to split up one commit into
multiple commits by moving parts of the changes of the original commit
out into a separate commit. This is quite an involved operation though:

  1. Identify the commit in question that is to be dropped.

  2. Perform an interactive rebase on top of that commit's parent.

  3. Modify the instruction sheet to "edit" the commit that is to be
     split up.

  4. Drop the commit via "git reset HEAD~".

  5. Stage changes that should go into the first commit and commit it.

  6. Stage changes that should go into the second commit and commit it.

  7. Finalize the rebase.

This is quite complex, and overall I would claim that most people who
are not experts in Git would struggle with this flow.

Introduce a new "split" subcommand for git-history(1) to make this way
easier. All the user needs to do is to say `git history split $COMMIT`.
From hereon, Git asks the user which parts of the commit shall be moved
out into a separate commit and, once done, asks the user for the commit
message. Git then creates that split-out commit and applies the original
commit on top of it.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

1 files changed, 62 insertions, 0 deletions

diff --git a/Documentation/git-history.adoc b/Documentation/git-history.adoc
index cc019de697..24dc907033 100644
--- a/Documentation/git-history.adoc
+++ b/Documentation/git-history.adoc
@@ -9,6 +9,7 @@ SYNOPSIS
 --------
 [synopsis]
 git history reword <commit> [--dry-run] [--update-refs=(branches|head)]
+git history split <commit> [--dry-run] [--update-refs=(branches|head)] [--] [<pathspec>...]
 
 DESCRIPTION
 -----------
@@ -57,6 +58,26 @@ The following commands are available to rewrite history in different ways:
 	details of this commit remain unchanged. This command will spawn an
 	editor with the current message of that commit.
 
+`split <commit> [--] [<pathspec>...]`::
+	Interactively split up <commit> into two commits by choosing
+	hunks introduced by it that will be moved into the new split-out
+	commit. These hunks will then be written into a new commit that
+	becomes the parent of the previous commit. The original commit
+	stays intact, except that its parent will be the newly split-out
+	commit.
++
+The commit messages of the split-up commits will be asked for by launching
+the configured editor. Authorship of the commit will be the same as for the
+original commit.
++
+If passed, _<pathspec>_ can be used to limit which changes shall be split out
+of the original commit. Files not matching any of the pathspecs will remain
+part of the original commit. For more details, see the 'pathspec' entry in
+linkgit:gitglossary[7].
++
+It is invalid to select either all or no hunks, as that would lead to
+one of the commits becoming empty.
+
 OPTIONS
 -------
 
@@ -72,6 +93,47 @@ OPTIONS
 	descendants of the original commit will be rewritten. With `head`, only
 	the current `HEAD` reference will be rewritten. Defaults to `branches`.
 
+EXAMPLES
+--------
+
+Split a commit
+~~~~~~~~~~~~~~
+
+----------
+$ git log --stat --oneline
+3f81232 (HEAD -> main) original
+ bar | 1 +
+ foo | 1 +
+ 2 files changed, 2 insertions(+)
+
+$ git history split HEAD
+diff --git a/bar b/bar
+new file mode 100644
+index 0000000..5716ca5
+--- /dev/null
++++ b/bar
+@@ -0,0 +1 @@
++bar
+(1/1) Stage addition [y,n,q,a,d,p,?]? y
+
+diff --git a/foo b/foo
+new file mode 100644
+index 0000000..257cc56
+--- /dev/null
++++ b/foo
+@@ -0,0 +1 @@
++foo
+(1/1) Stage addition [y,n,q,a,d,p,?]? n
+
+$ git log --stat --oneline
+7cebe64 (HEAD -> main) original
+ foo | 1 +
+ 1 file changed, 1 insertion(+)
+d1582f3 split-out commit
+ bar | 1 +
+ 1 file changed, 1 insertion(+)
+----------
+
 GIT
 ---
 Part of the linkgit:git[1] suite