Fix "invalid version number" for untagged tesseract versions

Fixes #770

src/ocrmypdf/_exec/tesseract.py

@@ -61,6 +61,7 @@ class TesseractVersion(StrictVersion):
         ^(\d+) \. (\d+) (\. (\d+))?  # groups: 1/major, 2/minor, 3/[skip], 4/patch
         [-]?  # optional hyphen separator
         (?:(alpha|beta|rc|dev)[.\-\ ]?(\d+)?)?  # 5/prerelease, 6/prerelease_num
+        (?:-(\d+)-g[0-9a-f]+)?  # untagged git version
         $
         ''',
         re.VERBOSE | re.ASCII,

tests/test_validation.py

@@ -237,6 +237,13 @@ def test_version_comparison():
         need_version='4.0.0',
         version_parser=TesseractVersion,
     )
+    vd.check_external_program(
+        program="tesseract",
+        package="tesseract",
+        version_checker=lambda: '4.1.1-rc2-25-g9707',
+        need_version='4.0.0',
+        version_parser=TesseractVersion,
+    )
     with pytest.raises(MissingDependencyError):
         vd.check_external_program(
             program="dummy_fails",