Improve version message when compiled from release archive (#186)

Author: ankane

src/slivarpkg/version.nim

@@ -1,2 +1,4 @@
+import strutils
+
 const slivarVersion* = "0.3.4"
-const slivarGitCommit* = staticExec("git rev-parse --verify HEAD")
+const slivarGitCommit* = staticExec("git rev-parse --verify HEAD 2>/dev/null").strip()