Add getCwd

It's nice for this to be a separate function and not just inline in
`absPath`.

Prepared as part of cl/1865, though I don't think I actually ended up
using it there.

Change-Id: I24d9d4a984cee0af587010baf04b3939a1c147ec

src/libutil/file-system.cc

@@ -17,15 +17,19 @@ namespace fs = std::filesystem;
 
 namespace nix {
 
+Path getCwd() {
+    char buf[PATH_MAX];
+    if (!getcwd(buf, sizeof(buf))) {
+        throw SysError("cannot get cwd");
+    }
+    return Path(buf);
+}
+
 Path absPath(Path path, std::optional<PathView> dir, bool resolveSymlinks)
 {
     if (path.empty() || path[0] != '/') {
         if (!dir) {
-            char buf[PATH_MAX];
+            path = concatStrings(getCwd(), "/", path);
-            if (!getcwd(buf, sizeof(buf))) {
-                throw SysError("cannot get cwd");
-            }
-            path = concatStrings(buf, "/", path);
         } else {
             path = concatStrings(*dir, "/", path);
         }

src/libutil/file-system.hh

@@ -29,6 +29,13 @@ namespace nix {
 struct Sink;
 struct Source;
 
+/**
+ * Get the current working directory.
+ *
+ * Throw an error if the current directory cannot get got.
+ */
+Path getCwd();
+
 /**
  * @return An absolutized path, resolving paths relative to the
  * specified directory, or the current directory otherwise.  The path