git: Make commit-last-msg available as alias

Move `git-commit-last-msg` into an autoloadable function. This way it
can also be executed as external script and thus in a git alias. This
makes it additionally possible to call it in vim over fugitive's `:Git`.

.config/zsh/autoload/git/git-commit-last-msg

@@ -0,0 +1,12 @@
+#!/usr/bin/env zsh
+
+# Commit, but put the last written commit message into the editor buffer.
+# For this it uses .git/COMMIT_EDITMSG but deletes all lines after and including
+# the first comment of the file.
+# Useful for example when the commit-msg hook fails but only slight
+# modifications are needed.
+# Additional arguments (for example `-n` to bypass the hooks) can be passed.
+
+local gitdir
+gitdir="$(git rev-parse --git-dir)" || return
+git commit -eF <(sed -n '/^#/q;p' "$gitdir/COMMIT_EDITMSG") "$@"