Handle edge cases

- When not in a git repo
- When the number of changes ends with a zero and is not zero
- Clarify rebase apply vs rebase merge. These are two different rebase
  backends. Apply uses git am and merge uses git cherry-pick. The merge
  backend is the newer and is being migrated to. Rebase apply is being
  deprecated.

Author: NonlinearFruit

scripts/git-prompt

@@ -1,33 +1,51 @@
 #!/bin/env sh
 
-# <https://stackoverflow.com/a/59115583>
-rebasing-branch() {
-  for location in rebase-merge rebase-apply; do
-    path=$(git rev-parse --git-path ${location})
-    if test -d ${path}; then
-      revision=$(<${path}/head-name)
-      echo ${revision##refs/heads/}
-      return 0
-    fi
-  done
+is_git_repo() {
+  git rev-parse --is-inside-work-tree > /dev/null 2>&1
 }
 
 branch() {
   git symbolic-ref HEAD --short 2>/dev/null
 }
 
+# <https://stackoverflow.com/a/59115583>
+rebase-branch-old() {
+  path=$(git rev-parse --git-path rebase-apply) # rebase.backend is apply
+  if test -d ${path}; then
+    revision=$(<${path}/head-name)
+    echo ${revision##refs/heads/}
+  fi
+}
+
+# <https://stackoverflow.com/a/59115583>
+rebase-branch-new() {
+  path=$(git rev-parse --git-path rebase-merge) # rebase.backend is merge
+  if test -d ${path}; then
+    revision=$(<${path}/head-name)
+    echo ${revision##refs/heads/}
+  fi
+}
+
 changes() {
   STAGED="$(git diff --cached --name-only 2>/dev/null | wc --lines)s"
   WORKTREE="$(git diff --name-only 2>/dev/null | wc --lines)w"
   UNTRACKED="$(git ls-files --others  --exclude-standard 2>/dev/null | wc --lines)u"
-  echo "${STAGED}${WORKTREE}${UNTRACKED}" | sed "s/0.//g"
+  echo "${STAGED}${WORKTREE}${UNTRACKED}" | sed -E ':1; s/(^|[^0-9])0[swu]/\1/g; t1;'
 }
 
+if ! is_git_repo; then
+  exit
+fi
+
 PROMPT=$(branch)
-if git rev-parse --quiet --verify MERGE_HEAD > /dev/null; then
+REBASE_BRANCH=$(rebase-branch-new)
+REBASE_BRANCH_OLD=$(rebase-branch-old)
+if [[ -n "$REBASE_BRANCH" ]]; then
+  PROMPT="$REBASE_BRANCH [rebasing]"
+elif [[ -n "$REBASE_BRANCH_OLD" ]]; then
+  PROMPT="$REBASE_BRANCH_OLD [*rebasing]"
+elif git rev-parse --quiet --verify MERGE_HEAD > /dev/null 2>&1; then
   PROMPT="$PROMPT [merging]"
-elif git rev-parse --quiet --verify REBASE_HEAD > /dev/null; then
-  PROMPT="$(rebasing-branch) [rebasing]"
 fi
 CHANGES=$(changes)
 if [[ -n "$CHANGES" ]]; then