tests: allow Google's internal builds of clang-format to be used stable

tests: allow Google's internal builds of clang-format to be used

These builds do not actually include any Google-specific formatting changes; the
only reason they don't include the LLVM version number is due to a decision to
elide the version number from *all* LLVM/clang projects.

For most builds of clang-format, even "unofficial" ones, the LLVM version will
be displayed; example:

```
clang-format version 14.0.0 (https://github.com/llvm/llvm-project.git 1830ec94ac022ae0b6d6876fc2251e6b91e5931e)
```

The Google-internal build looks like this:

```
clang-format version google3-trunk (1830ec94ac022ae0b6d6876fc2251e6b91e5931e)
```

Differential Revision: https://phab.mercurial-scm.org/D10538

--- a/tests/hghave.py	Tue Oct 26 18:53:58 2021 +0530
+++ b/tests/hghave.py	Fri Apr 30 16:00:40 2021 -0700
@@ -611,7 +611,14 @@
 def has_clang_format():
     m = matchoutput('clang-format --version', br'clang-format version (\d+)')
     # style changed somewhere between 10.x and 11.x
-    return m and int(m.group(1)) >= 11
+    if m:
+        return int(m.group(1)) >= 11
+    # Assist Googler contributors, they have a centrally-maintained version of
+    # clang-format that is generally very fresh, but unlike most builds (both
+    # official and unofficial), it does *not* include a version number.
+    return matchoutput(
+        'clang-format --version', br'clang-format .*google3-trunk \([0-9a-f]+\)'
+    )
 
 
 @check("jshint", "JSHint static code analysis tool")