Refactor AddFile(s) and add ReadFile

Changed AddFile(s) function signatures to be more consistent with other repo_harness
functions. Also added support for nested files to AddFile(s). Implemented ReadFile,
which reads a file from the appropriate remote project.

BUG=chromium:980346
TEST=run_tests.sh

Change-Id: I2bec09479c427971ead6ee9d536cd7cc8a37283d
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/infra/go/+/1718740
Commit-Queue: Jack Neus <jackneus@google.com>
Tested-by: Jack Neus <jackneus@google.com>
Reviewed-by: Evan Hernandez <evanhernandez@chromium.org>
diff --git a/internal/git/git.go b/internal/git/git.go
index 1266531..0c03d97 100644
--- a/internal/git/git.go
+++ b/internal/git/git.go
@@ -39,6 +39,13 @@
 	return cmdOutput, err
 }
 
+// RunGitIgnore the specified git command in the specified repo and returns
+// only an error, not the command output.
+func RunGitIgnoreOutput(gitRepo string, cmd []string) error {
+	_, err := RunGit(gitRepo, cmd)
+	return err
+}
+
 // GetCurrentBranch returns current branch of a repo, and an empty string
 // if repo is on detached HEAD.
 func GetCurrentBranch(cwd string) string {