fix: do not call git checkout unless necessary

Lake/Config/Resolve.lean

@@ -21,8 +21,9 @@ cloning and/or updating it as necessary.
 def materializeGit
 (name : String) (dir : FilePath) (url rev : String) : (LogT IO) PUnit := do
   if ← dir.isDir then
-    logInfo s!"{name}: trying to update {dir} to revision {rev}"
     let hash ← parseOriginRevision rev dir
+    if (← headRevision dir) == hash then return
+    logInfo s!"{name}: trying to update {dir} to revision {rev}"
     unless ← revisionExists hash dir do fetch dir
     checkoutDetach hash dir
   else

Lake/Util/Git.lean

@@ -10,6 +10,9 @@ namespace Lake.Git
 def upstreamBranch :=
   "master"
 
+def isFullObjectName (rev : String) : Bool :=
+  rev.length == 40 && rev.all fun c => c.isDigit || ('a' <= c && c <= 'f')
+
 def defaultRevision : Option String → String
   | none => upstreamBranch
   | some branch => branch
@@ -51,7 +54,8 @@ def parseRevision (rev : String) (repo : Option FilePath := none) : IO String :=
 def headRevision (repo : Option FilePath := none) : IO String :=
   parseRevision "HEAD" repo
 
-def parseOriginRevision (rev : String) (repo : Option FilePath := none) : IO String :=
+def parseOriginRevision (rev : String) (repo : Option FilePath := none) : IO String := do
+  if isFullObjectName rev then return rev
   (parseRevision ("origin/" ++ rev) repo) <|> parseRevision rev repo
     <|> throw (IO.userError s!"cannot find revision {rev} in repository {repo}")