scm-prompt: fix _dotfiles_scm_info

Summary: I was using """_dotfiles_scm_info '[%s] '""" in my prompt, which broke after the refactoring.  Add a unit test and fix the code.

Test Plan: wrote a test for the breakage, edited code until they all passed

Reviewers: rmcelroy, tja

Reviewed By: rmcelroy

Subscribers: mjpieters, #pe_security

Differential Revision: https://phabricator.intern.facebook.com/D4945132

Signature: t1:4945132:1493136425:4516151982d173584cf874985de9de1fed0962d5

scripts/scm-prompt.sh

@@ -208,10 +208,13 @@ _scm_prompt()
 
 _dotfiles_scm_info() {
   local fmt
+  fmt=$1
   if [[ -z "$fmt" ]]; then
     if [[ -n "$WANT_OLD_SCM_PROMPT" ]]; then
       fmt="%s"
+    else
+      fmt=' (%s)'
     fi
   fi
-  _scm_prompt $fmt
+  _scm_prompt "$fmt"
 }

tests/test-scm-prompt-compat.t

@@ -26,3 +26,12 @@ Test old mode
   5cad84d (no-eol)
   $ cmd hg book active
   active (no-eol)
+
+Test format string
+  $ oldcmd() {
+  >   "$@"
+  >   _dotfiles_scm_info "g g %s g g\n"
+  > }
+  $ hg init repo
+  $ oldcmd cd repo
+  g g empty g g