Extract shared prompt values and refactor bash/git prompts

Move color constants and PS1 escape sequences from bash-prompt.sh into
a new shared rc/03-values.sh, and extract the user@host:dir fragment as
LWS_PS1_USER_HOST to eliminate duplication between bash-prompt.sh and
git-prompt.sh. Wire up git-prompt to use bash-git-prompt with the shared
prompt variables.

rc/git-prompt.sh

@@ -9,5 +9,8 @@ GIT_PROMPT_INIT="$GIT_PROMPT_PATH/gitprompt.sh"
 if [ -f "$GIT_PROMPT_INIT" ]; then
   export GIT_PROMPT_PATH
   GIT_PROMPT_ONLY_IN_REPO=1
+  GIT_PROMPT_USER_HOST="${LWS_PS1_USER_HOST}"
+  GIT_PROMPT_START_USER="_LAST_COMMAND_INDICATOR_ ${GIT_PROMPT_USER_HOST}"
+  GIT_PROMPT_START_ROOT="${GIT_PROMPT_START_USER}"
   source "$GIT_PROMPT_INIT"
 fi