presubmit: generate a diff with prefix a/ and b/

Historically, git_cl.py and presubmit_support.py generated diffs
inconsistently.

==== git_cl.py ====
1) git_cl.py:_RunClangFormatDiff() assumed the input diff was
generated with --no-prefix. Therefore, it hard-coded -p0 as a param
for clang_format_diff.py. If the diff was generated without --no-prefix,
it would have to pass -p1 instead.

2) git_cl.py _RunYapf() and _RunGoogleJavaFormat assumed that
the input diff was generated with prefixes. Therefore,
it parsed the diffs, assuming that the diff was generated with
a/ and b/ prefixes.

This discrepancy wasn't an issue because each of the Run functions
generated and consumed the diffs within themselves. It became an issue
when https://crrev.com/c/6931775 was developed. The CL consolidated
the diff generation into a common function, so that all _Run functions
need to agree with the presence of the prefixes.

The CL changed _RunClangFormatDiff() to use -p1 instead of -p0,
mainly because
- it's the p level for the default git-diff option
- it's the p level for the unix diff util
- CiderG Chromium also generates the diff with the prefixes to
emulate the default diff output.
- I highly believe that --no-prefix was chosen for no reasons.
It seems to be a random choice to work with -p0.
- Either Java/Python or Clang wrapper should be changed.

==== presubmit_support.py ====

presubmit_support.py can be given a diff_file via --diff_file.
Otherwise, it generates the diff based on --upstream_commit.

a) it doesn't enforce the presence of the prefixes in the input
diff_file. As stated above, the Chromium extension generates
a diff with prefixes.

b) in contrast, when it is not given --diff_file, it generates
the diff with --no-prefix, and I don't find any reasons for it.
I believe that it's from a copy-pasted random choice.

==== the problem ====

This discarepancy became a problem, when crrev.com/c/6937365
was landed. It enforces git_cl.py and presubmit_support.py to agree
for the format of the input diff. That is, the presence of
the prefixes must be agreed by all the following.
- presubmit_support.py with --diff_file /tmp/abc
- presubmit_suppory.py without --diff_file /tmp/abc
- git cl format
- git cl format --input_diff_file /tmp/abc

==== possible solutions ====
Obviously, there are 3 choices.

1) update the regex and parsers of git_cl.py to auto detect the best
-p{num}.
This was the least preferred option, as it can be fragile.

2) update Chromium extension and git_cl.py to use --no-prefix.
i.e.,
- update the Chromium CiderG to generate diffs without the prefix.
- update the regex used in _ComputeFormatDiffLineRanges() to assume
that the input is generated with --no-prefix.
- change _RunClangFormatDiff() to pass -p0 instead of -p1.

3) update presubmit_support.py to generate diff without --no-prefix.

==== What this CL does ====
It implements (3).
- That is the default option for git-diff.
- I don't find obvious reasons to use --no-prefix.
- CiderG has been generating diffs with prefixes for weeks.
- It implies that presubmit_support.py works fine with a prefixed diff.

Bug: 386840832
Change-Id: Iac8a4fc30f101e70e3ccc56f9f8ee48198dfa833
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/6939737
Commit-Queue: Scott Lee <ddoman@chromium.org>
Reviewed-by: Gavin Mak <gavinmak@google.com>

presubmit_support.py

@@ -1059,7 +1059,8 @@ class _GitDiffCache(_DiffCache):
                                                 files=[],
                                                 full_move=True,
                                                 branch=self._upstream,
-                                                branch_head=self._end_commit)
+                                                branch_head=self._end_commit,
+                                                allow_prefix=True)
             # 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)