Downstream repository is checked for uncommitted changes and commits …

…made in detached HEAD state when `update`ing/`restore`ing Git dependency

Checking for uncommitted changes prevents hard-to-track bugs as well as non-repeatable builds that might occur by merging user's changes into the new working directory.
Checking for detached commits makes working in the downstream repository safer - it saves the user from having to inspect reflog after "losing" her commits.
It also forces the user to move the changes to a permanent branch when making meaningful changes.

src/Paket.Core/GitHandling.fs

@@ -158,6 +158,20 @@ let fetchCache repoCacheFolder cloneUrl =
     with
     | exn -> failwithf "Fetching the git cache at %s failed.%sMessage: %s" repoCacheFolder Environment.NewLine exn.Message
 
+let checkForUncommittedChanges repoFolder =
+    try
+        tracefn "Checking for uncommitted changes in %s" repoFolder
+        CommandHelper.gitCommand repoFolder "diff-index --quiet HEAD --" |> ignore
+    with
+    | exn -> failwithf "It seems there are uncommitted changes in the repository. The changes must be committed/discarded first.%sMessage: %s" Environment.NewLine exn.Message
+
+let checkForCommitsMadeInDetachedHeadState repoFolder =
+    try
+        tracefn "Checking for commits made in detached HEAD state in %s" repoFolder
+        CommandHelper.gitCommand repoFolder "name-rev --no-undefined HEAD --" |> ignore
+    with
+    | exn -> failwithf "It seems that some commits would become unreachable after checkout. Create branch for the commits or reset them first and try again.%sMessage: %s" Environment.NewLine exn.Message
+
 let tagCommitForCheckout repoFolder commit =
     try
         CommandHelper.runSimpleGitCommand repoFolder (sprintf "tag -f %s %s" paketCheckoutTag commit) |> ignore
@@ -181,6 +195,9 @@ let checkoutToPaketFolder repoFolder cloneUrl cacheCloneUrl commit =
             CommandHelper.runSimpleGitCommand destination (sprintf "clone %s %s" (quote cacheCloneUrl) (quote repoFolder)) |> ignore
             CommandHelper.runSimpleGitCommand repoFolder (sprintf "remote set-url origin %s" <| quote cloneUrl) |> ignore
 
+        checkForUncommittedChanges repoFolder
+        checkForCommitsMadeInDetachedHeadState repoFolder
+
         tracefn "Setting %s to %s" repoFolder commit
         tagCommitForCheckout repoFolder commit
         checkoutTaggedCommit repoFolder