presubmit: use git diff -U0 for CheckPatchFormatted()

git cl format uses `git diff -U0` when it has to find the files
and ranges to format. Similarly, CiderG generates diffs with
-U0 and run presubmits for the checks.

However, the scm.py:GenerateDiff(), which CheckPatchFormatted() uses,
doesn't specify -U and it defaults to -U3.

This CL updates CheckPatchFormatted() to pass -U0 to
scm.py:GenerateDiff() so that Presubmit and git cl format are
always given diffs generated with -U0.

Bug: 386840832
Change-Id: I19479ab139b7dba8ba4e5e5fed89eca4d2e66412
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/6952890
Auto-Submit: Scott Lee <ddoman@chromium.org>
Commit-Queue: Brian Ryner <bryner@google.com>
Commit-Queue: Scott Lee <ddoman@chromium.org>
Reviewed-by: Brian Ryner <bryner@google.com>

presubmit_support.py

@@ -1060,7 +1060,8 @@ class _GitDiffCache(_DiffCache):
                                                 full_move=True,
                                                 branch=self._upstream,
                                                 branch_head=self._end_commit,
-                                                allow_prefix=True)
+                                                allow_prefix=True,
+                                                context=0)
             # Compute a single diff for all files and parse the output; with git
             # this is much faster than computing one diff for each file.
             self._diffs_by_file = _parse_unified_diff(unified_diff)

scm.py

@@ -1021,7 +1021,8 @@ class GIT(object):
                      branch_head: str = 'HEAD',
                      full_move: bool = False,
                      files: Optional[Iterable[str]] = None,
-                     allow_prefix: bool = False) -> str:
+                     allow_prefix: bool = False,
+                     context: int = 3) -> str:
         """Diffs against the upstream branch or optionally another branch.
 
         full_move means that move or copy operations should completely recreate the
@@ -1036,6 +1037,7 @@ class GIT(object):
             '-p',
             '--no-color',
             '--no-ext-diff',
+            f'-U{context}',
         ]
 
         if allow_prefix:

tests/presubmit_unittest.py

@@ -420,7 +420,8 @@ class PresubmitUnittest(PresubmitTestsBase):
                                                      full_move=True,
                                                      branch='upstream',
                                                      branch_head='end_commit',
-                                                     allow_prefix=True)
+                                                     allow_prefix=True,
+                                                     context=0)
 
 
         f_blat = os.path.normpath('boo/blat.cc')