buildx/util/gitutil
Justin Chadwell 48d7dafbd5 git: update gitutil test utilities
- Adds a new GitServeHTTP function to start an http server to serve a
  target git repository.
- Adds a new GitDir helper method to get the path to the .git
  directory
- Updates the GitAdd method to take a variable number of files

Signed-off-by: Justin Chadwell <me@jedevc.com>
..
gitpath_unix.go build: lookup the right git binary on WSL
gitpath_windows.go build: lookup the right git binary on WSL
gitutil.go git: update gitutil test utilities
gitutil_test.go build: strip credentials from remote url on collecting Git provenance info
testutil.go git: update gitutil test utilities
testutilserve.go git: update gitutil test utilities