Add git-spinoff command

README.md

@@ -27,6 +27,7 @@ Everyday helpful commands:
 * [git-branches-containing](#git-branches-containing)
 * [git-recent-branches](#git-recent-branches)
 * [git-remote-branches](#git-remote-branches)
+* [git-remote-tracking-branch](#git-remote-tracking-branch)
 * [git-repo](#git-repo)
 * [git-root](#git-root)
 * [git-initial-commit](#git-initial-commit)
@@ -38,6 +39,7 @@ Everyday helpful commands:
 * ⭐️ [git-modified](#git-modified)
 * ⭐️ [git-modified-since](#git-modified-since)
 * ⭐️ [git-separator](#git-separator)
+* ⭐️ [git-spinoff](#git-spinoff)
 
 Statistics:
 
@@ -146,6 +148,19 @@ rebase operations.  (They should be used as a temporary measure, and ideally
 taken out of the history again when done rebasing.)
 
 
+### git spinoff
+
+Inspired by Magit's `spinoff` command.  Creates and checks out
+a new branch starting at and tracking the current branch.  That
+branch in turn is reset to the last commit it shares with its
+upstream.  If the current branch has no upstream or no unpushed
+commits, then the new branch is created anyway and the previously
+current branch is not touched.
+
+This is useful to create a feature branch after work has already
+began on the old branch (likely but not necessarily "master").
+
+
 ### git push-current
 
 Pushed the current branch out to `origin`, and makes sure to setup tracking of
@@ -191,6 +206,12 @@ Returns a list of local branches, ordered by recency:
     qux
 
 
+### git remote-tracking-branch
+
+Print the name of the remote tracking branch of the current or
+given local branch name, if one exists.  Errors otherwise.
+
+
 ### git local-commits / git has-local-commits
 
 Returns a list of commits that are still in your local repo, but haven't been

git-remote-tracking-branch

@@ -0,0 +1,31 @@
+#!/bin/sh
+set -eu
+
+usage () {
+    echo "usage: git remote-tracking-branch [-h] [<branch>]" >&2
+    echo >&2
+    echo "Prints the fully qualified name of the remote tracking" >&2
+    echo "branch for the given local branch name." >&2
+    echo >&2
+    echo "Options:" >&2
+    echo "-h    Show this help" >&2
+}
+
+while getopts h flag; do
+    case "$flag" in
+        h) usage; exit 2;;
+    esac
+done
+shift $(($OPTIND - 1))
+
+if [ $# -gt 1 ]; then
+  usage
+  exit 2
+fi
+
+branch="${1:-}"
+
+# This result is constructed so that it only prints the output of
+# git-rev-parse if the command succeeded, and is silent otherwise
+result="$(git rev-parse --symbolic-full-name --abbrev-ref "${branch}@{u}" 2>/dev/null)"
+echo "$result"

git-spinoff

@@ -0,0 +1,65 @@
+#!/bin/sh
+set -eu
+
+#
+# Inspired by Magit's super useful `magit-branch-spinoff` command.
+# See also https://magit.vc/manual/magit/Branch-Commands.html
+#
+
+usage () {
+    echo "usage: git spinoff [-h] <new-name> [<base>]" >&2
+    echo >&2
+    echo "Creates and checks out a new branch starting at and tracking the" >&2
+    echo "current branch.  That branch in turn is reset to the last commit it" >&2
+    echo "shares with its upstream.  If the current branch has no upstream or no" >&2
+    echo "unpushed commits, then the new branch is created anyway and the" >&2
+    echo "previously current branch is not touched." >&2
+    echo >&2
+    echo "This is useful to create a feature branch after work has already" >&2
+    echo "began on the old branch (likely but not necessarily \"master\")." >&2
+    echo >&2
+    echo "Options:" >&2
+    echo "-h    Show this help" >&2
+}
+
+while getopts h flag; do
+    case "$flag" in
+        h) usage; exit 2;;
+    esac
+done
+shift $(($OPTIND - 1))
+
+if [ $# -lt 1 -o $# -gt 2 ]; then
+  usage
+  exit 2
+fi
+
+new_name="$1"
+rawbase="${2:-}"
+if [ -z "$rawbase" ]; then
+    base="$(git current-branch)"
+else
+    base="$rawbase"
+fi
+
+base_sha="$(git sha -s "$base")"
+
+#
+# NOTE:
+# The flag -B is the transactional equivalent of
+#     $ git branch -f <branch> [<start point>]
+#     $ git checkout <branch>
+#
+git checkout -q --track -B "$new_name" "$base"
+
+rtb="$(git remote-tracking-branch "$base")"
+if [ -n "$rtb" ]; then
+    merge_base="$(git merge-base "$base" "$rtb")"
+    git branch -vf "$base" "$merge_base"
+fi
+
+if [ "$(git sha -s "$base")" != "$base_sha" ]; then
+    echo "$base reset to $(git sha -s "$base") (was $base_sha)"
+else
+    echo "$base not touched"
+fi