git:commit-last-msg: Make more robust

Use core.commentchar to identify commented lines and use the cut line
instead of just deleting from the first comment on, as this would break
for example in git generated messages (e.g. squashes).

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

@@ -1,12 +1,22 @@
 #!/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.
+# Commit, but put the last written commit message into the editor buffer. For
+# this it uses .git/COMMIT_EDITMSG but deletes all commented lines and
+# everything from the cut line on (i.e. `-- >8 --`). All arguments are passed to
+# git-commit.
+#
 # 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.
+# modifications are needed. For example, just run:
+#
+#     git-commit-last-msg --no-verify --no-edit
+
+local gitdir cut_line cchar
+cut_line='------------------------ >8 ------------------------'
 
-local gitdir
 gitdir="$(git rev-parse --git-dir)" || return
-git commit -eF <(sed -n '/^#/q;p' "$gitdir/COMMIT_EDITMSG") "$@"
+
+cchar="$(git config --get --default='#' core.commentchar)"
+# only ^ needs escaping because of the character class used
+cchar="${cchar//^/\\^}"
+
+git commit -eF <(sed -n "/$cut_line/q; /^[$cchar]/!p" "$gitdir/COMMIT_EDITMSG") "$@"