Record the commits that were compared

Record the commit ID in addition to the symbolic name of the version
being tested. This makes it easier to figure out what has been
compared when reading logs that don't always indicate explicitly what
things like HEAD are.

This makes the title of HTML reports somewhat verbose, but I think
that's a small price to pay.

scripts/abi_check.py

@@ -106,6 +106,12 @@ class AbiChecker(object):
             stderr=subprocess.STDOUT
         )
         self.log.debug(worktree_output.decode("utf-8"))
+        version.commit = subprocess.check_output(
+            [self.git_command, "rev-parse", worktree_rev],
+            cwd=git_worktree_path,
+            stderr=subprocess.STDOUT
+        ).decode("ascii").rstrip()
+        self.log.debug("Commit is {}".format(version.commit))
         return git_worktree_path
 
     def _update_git_submodules(self, git_worktree_path, version):
@@ -161,6 +167,13 @@ class AbiChecker(object):
                     os.path.join(root, file)
                 )
 
+    @staticmethod
+    def _pretty_revision(version):
+        if version.revision == version.commit:
+            return version.revision
+        else:
+            return "{} ({})".format(version.revision, version.commit)
+
     def _get_abi_dumps_from_shared_libraries(self, version):
         """Generate the ABI dumps for the specified git revision.
         The shared libraries must have been built and the module paths
@@ -175,7 +188,7 @@ class AbiChecker(object):
                 "abi-dumper",
                 module_path,
                 "-o", output_path,
-                "-lver", version.revision
+                "-lver", self._pretty_revision(version),
             ]
             abi_dump_output = subprocess.check_output(
                 abi_dump_command,
@@ -224,7 +237,10 @@ class AbiChecker(object):
         """Generate a report of the differences between the reference ABI
         and the new ABI. ABI dumps from self.old_version and self.new_version
         must be available."""
-        compatibility_report = ""
+        compatibility_report = ("Checking evolution from {} to {}\n".format(
+            self._pretty_revision(self.old_version),
+            self._pretty_revision(self.new_version)
+        ))
         compliance_return_code = 0
         shared_modules = list(set(self.old_version.modules.keys()) &
                               set(self.new_version.modules.keys()))
@@ -371,6 +387,7 @@ def run_main():
             version="old",
             repository=abi_args.old_repo,
             revision=abi_args.old_rev,
+            commit=None,
             crypto_repository=abi_args.old_crypto_repo,
             crypto_revision=abi_args.old_crypto_rev,
             abi_dumps={},
@@ -380,6 +397,7 @@ def run_main():
             version="new",
             repository=abi_args.new_repo,
             revision=abi_args.new_rev,
+            commit=None,
             crypto_repository=abi_args.new_crypto_repo,
             crypto_revision=abi_args.new_crypto_rev,
             abi_dumps={},