[PATCH v3 16/27] rebase -i: teach do_pick the option --amend

[Fabian Ruch <bafain@gmail.com>]

`do_pick` is the git-cherry-pick wrapper in git-rebase--interactive
that is used to implement the to-do list commands `pick`, `reword`
and `edit`. To cater for the different pick behaviours (like
`squash`), `do_pick` accepts several options not only from the
git-cherry-pick but also the git-commit interface.

Add the option `--amend` from the git-commit interface to the options
pool of `do_pick`. It creates a new commit for the changes introduced
by the picked commit and the previous one. The previous commit is
then replaced with the new commit. If no other options are specified,
the log message of the previous commit is used.

Be careful when `--amend` is used to pick a root commit because HEAD
might point to the sentinel commit but there is still nothing to
amend. Be sure to initialize `amend` so that commits are squashed
even when git-rebase--interactive is interrupted for resolving
conflicts. It is not a mistake to do the initialization regardless of
any conflicts because `amend` is always cleared before the next to-do
item is processed.

Signed-off-by: Fabian Ruch <bafain@gmail.com>
---
 git-rebase--interactive.sh | 18 +++++++++++++++++-
 1 file changed, 17 insertions(+), 1 deletion(-)

diff --git a/git-rebase--interactive.sh b/git-rebase--interactive.sh
index a5a8aa3..20a637a 100644
--- a/git-rebase--interactive.sh
+++ b/git-rebase--interactive.sh
@@ -464,10 +464,16 @@ record_in_rewritten() {
 
 # Apply the changes introduced by the given commit to the current head.
 #
-# do_pick [--edit] <commit>
+# do_pick [--amend] [--edit] <commit>
 #
 # Wrapper around git-cherry-pick.
 #
+# --amend
+#     After picking <commit>, replace the current head commit with a new
+#     commit that also introduces the changes of <commit>.
+#
+#     _This is not a git-cherry-pick option._
+#
 # -e, --edit
 #     After picking <commit>, open an editor and let the user edit the
 #     commit message. The editor contents becomes the commit message of
@@ -489,6 +495,16 @@ do_pick () {
 	while test $# -gt 0
 	do
 		case "$1" in
+		--amend)
+			if test "$(git rev-parse HEAD)" = "$squash_onto" || ! git rev-parse -q --verify HEAD >/dev/null
+			then
+				warn "do_pick: nothing to amend"
+				return 2
+			fi
+			rewrite=y
+			rewrite_amend=y
+			git rev-parse --verify HEAD >"$amend"
+			;;
 		-e|--edit)
 			rewrite=y
 			rewrite_edit=y
-- 
2.0.1