tools/filetools.go: extract fastWalker consumer func

When calling FastWalkGitRepo, the implementation constructs an
instance of the *fastWalker type, and then consumes items from the
<-chan until it receives a close().

This was left mostly unchanged since 3b9e629d (use a single channel,
2016-11-29), when we had only the single function `FastWalkGitRepo`.

In a subsequent commit, we will introduce a new function
`FastWalkGitRepoAll` with does _not_ exclude patterns found in a
repository's .gitignore(s).

In preparation for this function, which we assume to share the
implementation of `FastWalkGitRepo` in its entirety (modulo the
construction of a `*fastWalker`), let's extract this fastWalkCallback
function to avoid repeating ourselves.

tools/filetools.go

@@ -152,7 +152,12 @@ type FastWalkCallback func(parentDir string, info os.FileInfo, err error)
 //
 // rootDir - Absolute path to the top of the repository working directory
 func FastWalkGitRepo(rootDir string, cb FastWalkCallback) {
-	walker := fastWalkWithExcludeFiles(rootDir, ".gitignore")
+	fastWalkCallback(fastWalkWithExcludeFiles(rootDir, ".gitignore"), cb)
+}
+
+// fastWalkCallback calls the FastWalkCallback "cb" for all files found by the
+// given *fastWalker, "walker".
+func fastWalkCallback(walker *fastWalker, cb FastWalkCallback) {
 	for file := range walker.ch {
 		cb(file.ParentDir, file.Info, file.Err)
 	}