Provide a more meaningful error-message for builtins.fetchGit if a …

…revision can't be checked out

A common pitfall when using e.g. `builtins.fetchGit` is the `fatal: not
a tree object`-error when trying to fetch a revision of a git-repository
that isn't on the `master` branch and no `ref` is specified.

In order to make clear what's the problem, I added a simple check
whether the revision in question exists and if it doesn't a more
meaningful error-message is displayed:

```
nix-repl> builtins.fetchGit { url = "https://github.com/owner/myrepo"; rev = "<commit not on master>"; }
moderror: --- Error ------------------------------------------------------------------------------------------------ nix
Cannot find git-revision in '<commit not on master>' of repo 'https://github.com/owner/myrepo' on ref 'master'!
```

Closes NixOS#2431

src/libfetchers/git.cc

@@ -322,6 +322,30 @@ struct GitInput : Input
         AutoDelete delTmpDir(tmpDir, true);
         PathFilter filter = defaultPathFilter;
 
+        RunOptions checkCommitOpts(
+            "git",
+            { "-C", repoDir, "cat-file", "commit", input->rev->gitRev() }
+        );
+        checkCommitOpts.searchPath = true;
+        checkCommitOpts.mergeStderrToStdout = true;
+
+        auto result = runProgram(checkCommitOpts);
+        if (WEXITSTATUS(result.first) == 128
+            && result.second.find("bad file") != std::string::npos
+        ) {
+            std::string msg = fmt("Cannot find git-revision in '" ANSI_YELLOW "%s" ANSI_NORMAL
+                    "' of repo '" ANSI_YELLOW "%s" ANSI_NORMAL "' on ref '" ANSI_YELLOW
+                    "%s" ANSI_NORMAL "!\n\n"
+                    "Please make sure that the " ANSI_BOLD "rev" ANSI_NORMAL " exists on the "
+                    ANSI_BOLD "ref" ANSI_NORMAL " you've specified or add " ANSI_BOLD
+                    "allRefs = true;" ANSI_NORMAL " to " ANSI_BOLD "fetchGit" ANSI_NORMAL ".",
+                input->rev->gitRev(),
+                actualUrl,
+                *input->ref);
+
+            throw Error(msg);
+        }
+
         if (submodules) {
             Path tmpGitDir = createTempDir();
             AutoDelete delTmpGitDir(tmpGitDir, true);