[Get] Use known version map to get appropriate tag.

 - Instead of recomputing out of thin are the expected tag for a version, use
   the map established in the previous commit.

Author: ddunbar

Sources/Get/RawClone.swift

@@ -65,10 +65,9 @@ class RawClone: Fetchable {
 
     /// contract, you cannot call this before you have attempted to `constrain` this clone
     func setCurrentVersion(_ ver: Version) throws {
-        let packageVersionsArePrefixed = repo.versionsArePrefixed
-        let v = (packageVersionsArePrefixed ? "v" : "") + ver.description
-        try Git.runCommandQuietly([Git.tool, "-C", path.asString, "reset", "--hard", v])
-        try Git.runCommandQuietly([Git.tool, "-C", path.asString, "branch", "-m", v])
+        let tag = repo.knownVersions[ver]!
+        try Git.runCommandQuietly([Git.tool, "-C", path.asString, "reset", "--hard", tag])
+        try Git.runCommandQuietly([Git.tool, "-C", path.asString, "branch", "-m", tag])
 
         print("Resolved version:", ver)
 

Sources/Utility/Git.swift

@@ -102,15 +102,6 @@ public class Git {
             return !(changes?.isEmpty ?? true)
         }
 
-        /**
-         - Returns: true if the package versions in this repository
-         are all prefixed with "v", otherwise false. If there are
-         no versions, returns false.
-         */
-        public var versionsArePrefixed: Bool {
-            return (try? Git.runPopen([Git.tool, "-C", path.asString, "tag", "-l"]))?.hasPrefix("v") ?? false
-        }
-
         public func fetch() throws {
 #if os(Linux)
             try system(Git.tool, "-C", path.asString, "fetch", "--tags", "origin", environment: ProcessInfo.processInfo().environment, message: nil)