hghave: add a check for pygit2

hghave: add a check for pygit2

This will make it easy to use availability of pygit2 as in conditions
such as `#if pygit2` and `(pygit2 !)`. It also makes it a little
clearer why the test case skipped (when that happens).

Differential Revision: https://phab.mercurial-scm.org/D8267

--- a/tests/hghave.py	Sat Mar 07 18:00:53 2020 -0500
+++ b/tests/hghave.py	Mon Mar 09 12:53:21 2020 -0700
@@ -381,6 +381,17 @@
     return (int(m.group(1)), int(m.group(2)))
 
 
+@check("pygit2", "pygit2 Python library")
+def has_git():
+    try:
+        import pygit2
+
+        pygit2.Oid  # silence unused import
+        return True
+    except ImportError:
+        return False
+
+
 # https://github.com/git-lfs/lfs-test-server
 @check("lfs-test-server", "git-lfs test server")
 def has_lfsserver():

--- a/tests/test-git-interop.t	Sat Mar 07 18:00:53 2020 -0500
+++ b/tests/test-git-interop.t	Mon Mar 09 12:53:21 2020 -0700
@@ -1,5 +1,4 @@
-This test requires pygit2:
-  > $PYTHON -c 'import pygit2' || exit 80
+#require pygit2
 
 Setup:
   > GIT_AUTHOR_NAME='test'; export GIT_AUTHOR_NAME