Check that the bottom PR is mergeable

GitHub lets you check if a PR is actually mergeable via the `gh` CLI by
retrieving the `mergeStateStatus` field. This field has a bunch of
values (see the [API documentation][1] linked below), but the one we care
about is `CLEAN`, indicating that the PR has no merge conflicts and has
passed PR checks.

I'd like to eventually add this information to `stack-pr view`, but it will
require some extra piping, and will slow down the command due to the
extra network request.

1: https://docs.github.com/en/graphql/reference/enums#mergestatestatus

src/stack_pr/cli.py

@@ -143,6 +143,13 @@
 
 If you are trying land the stack, please update it first by calling 'submit'.
 
+PR info from github: {d}
+"""
+ERROR_STACKINFO_PR_NOT_MERGEABLE = """Associated PR is not mergeable on GitHub!
+     {e}
+
+Please fix the issues on GitHub.
+
 PR info from github: {d}
 """
 ERROR_REPO_DIRTY = """There are uncommitted changes.
@@ -424,7 +431,7 @@ def set_base_branches(st: List[StackEntry], target: str):
 
 def verify(st: List[StackEntry], check_base: bool = False):
     log(h("Verifying stack info"), level=1)
-    for e in st:
+    for index, e in enumerate(st):
         if e.has_missing_info():
             error(ERROR_STACKINFO_MISSING.format(**locals()))
             raise RuntimeError
@@ -440,7 +447,7 @@ def verify(st: List[StackEntry], check_base: bool = False):
                 "view",
                 e.pr,
                 "--json",
-                "baseRefName,headRefName,number,state,body,title,url",
+                "baseRefName,headRefName,number,state,body,title,url,mergeStateStatus",
             ]
         )
         d = json.loads(ghinfo)
@@ -469,6 +476,11 @@ def verify(st: List[StackEntry], check_base: bool = False):
             error(ERROR_STACKINFO_PR_BASE_MISMATCH.format(**locals()))
             raise RuntimeError
 
+        # The first entry on the stack needs to be actually mergeable on GitHub.
+        if check_base and index == 0 and d["mergeStateStatus"] != "CLEAN":
+            error(ERROR_STACKINFO_PR_NOT_MERGEABLE.format(**locals()))
+            raise RuntimeError
+
 
 def print_stack(st: List[StackEntry], level=1):
     log(b("Stack:"), level=level)