bin: add git-amend-last

.local/bin/git-amend-last

@@ -0,0 +1,40 @@
+#!/usr/bin/env bash
+
+# This essentially makes the current commit empty so you can see the full diff
+# of the commit to be amended.
+#
+# This allows something similar to "Amend Last Commit" of git gui.
+
+set -euo pipefail
+
+old_commit=$(git rev-parse HEAD)
+
+git-fmt() {
+  git show -s --format="$1"
+}
+
+title=$(git-fmt %s)
+message=$(git-fmt %B)
+
+GIT_AUTHOR_NAME=$(git-fmt %an)
+GIT_AUTHOR_EMAIL=$(git-fmt %ae)
+GIT_AUTHOR_DATE=$(git-fmt %aD)
+GIT_COMMITTER_NAME=$(git-fmt %cn)
+GIT_COMMITTER_EMAIL=$(git-fmt %ce)
+GIT_COMMITTER_DATE=$(git-fmt %cD)
+
+export \
+  GIT_AUTHOR_NAME GIT_AUTHOR_EMAIL GIT_AUTHOR_DATE \
+  GIT_COMMITTER_NAME GIT_COMMITTER_EMAIL GIT_COMMITTER_DATE
+
+git reset --soft HEAD~1
+new_commit=$(git commit-tree -p HEAD 'HEAD^{tree}' -m "$message")
+
+git update-ref -m "amend-last: $title" HEAD "$new_commit"
+
+# If we're rebasing, git stores the current commit in rebase-merge/amend to
+# know whether --continue should result in an amend. If we don't update this,
+# --continue will complain that we have unstaged changes instead.
+if git rev-parse -q --verify rebase-merge/amend; then
+  git update-ref rebase-merge/amend "$new_commit" "$old_commit"
+fi