Make koch friendlier to offline environments (#24713)

(cherry picked from commit d94e535145)

koch.nim

@@ -171,7 +171,7 @@ proc bundleAtlasExe(latest: bool, args: string) =
   cloneDependency(distDir, "https://github.com/nim-lang/atlas.git",
                   commit = commit, allowBundled = true)
   cloneDependency(distDir / "atlas" / distDir, "https://github.com/nim-lang/sat.git",
-                commit = SatStableCommit, allowBundled = true)
+                  commit = SatStableCommit, allowBundled = true)
   # installer.ini expects it under $nim/bin
   nimCompile("dist/atlas/src/atlas.nim",
              options = "-d:release --noNimblePath -d:nimAtlasBootstrap " & args)

tools/deps.nim

@@ -4,10 +4,12 @@ import std/private/gitutils
 when defined(nimPreviewSlimSystem):
   import std/assertions
 
-proc exec(cmd: string) =
+proc tryexec(cmd: string): int =
   echo "deps.cmd: " & cmd
-  let status = execShellCmd(cmd)
-  doAssert status == 0, cmd
+  execShellCmd(cmd)
+
+proc exec(cmd: string) =
+  doAssert tryexec(cmd) == 0, cmd
 
 proc execRetry(cmd: string) =
   let ok = retryCall(call = block:
@@ -34,8 +36,10 @@ proc cloneDependency*(destDirBase: string, url: string, commit = commitHead,
     let oldDir = getCurrentDir()
     setCurrentDir(destDir)
     try:
-      execRetry "git fetch -q"
-      exec fmt"git checkout -q {commit}"
+      let checkoutCmd = fmt"git checkout -q {commit}"
+      if tryexec(checkoutCmd) != 0:
+        execRetry "git fetch -q"
+        exec checkoutCmd
     finally:
       setCurrentDir(oldDir)
   elif allowBundled: