list: add a --relative-to option

The current list output only shows project paths relative to the
root of the repo client checkout.  It can be helpful to also get
a listing of paths based on other paths (e.g. the current working
directory), so add an option to repo list to support that.  We'll
leverage this in bash completion to support completing projects by
their local paths and not just remote names.

Bug: https://crbug.com/gerrit/14797
Change-Id: Ia2b35d18c890217768448118b003874a1016efd4
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/312904
Reviewed-by: Xin Li <delphij@google.com>
Tested-by: Mike Frysinger <vapier@google.com>

completion.bash

@@ -37,6 +37,7 @@ __complete_repo_list_branches() {
 __complete_repo_list_projects() {
   local repo=${COMP_WORDS[0]}
   "${repo}" list -n 2>/dev/null
+  "${repo}" list -p --relative-to=. 2>/dev/null
 }
 
 # Complete the repo <command> argument.