[utils] support "Reverts ${PR}" in commit messages

[gburgessiv]

A bisection in ChromeOS ended at a reverted commit, which wasn't flagged by this revert checking script, since it used Reverts ${PR} rather than This reverts commit ${SHA}.

grep says that somewhere around 400 reverts in the last year have used Reverts ${PR} syntax. Support it.

Tested in part by running the command that was expected to catch this revert:

$ ./revert_checker.py -C ~/llvm/main/ \
    3b5e7c83a6e226d5bd7ed2e9b67449b64812074c origin/main \
    | grep -q 4b0276d1c9cb558f3c20736dce802ceb26c0b958
$ echo $?
0

[gburgessiv]

Thank you!

[llvm-ci]

LLVM Buildbot has detected a new failure on builder clang-aarch64-global-isel running on linaro-clang-aarch64-global-isel while building llvm at step 7 "ninja check 1".

Full details are available at: https://lab.llvm.org/buildbot/#/builders/125/builds/2841

Here is the relevant piece of the build log for the reference

Step 7 (ninja check 1) failure: stage 1 checked (failure)
******************** TEST 'lit :: googletest-timeout.py' FAILED ********************
Exit Code: 1

Command Output (stdout):
--
# RUN: at line 9
not env -u FILECHECK_OPTS "/usr/bin/python3.10" /home/tcwg-buildbot/worker/clang-aarch64-global-isel/llvm/llvm/utils/lit/lit.py -j1 --order=lexical -v Inputs/googletest-timeout    --param gtest_filter=InfiniteLoopSubTest --timeout=1 > /home/tcwg-buildbot/worker/clang-aarch64-global-isel/stage1/utils/lit/tests/Output/googletest-timeout.py.tmp.cmd.out
# executed command: not env -u FILECHECK_OPTS /usr/bin/python3.10 /home/tcwg-buildbot/worker/clang-aarch64-global-isel/llvm/llvm/utils/lit/lit.py -j1 --order=lexical -v Inputs/googletest-timeout --param gtest_filter=InfiniteLoopSubTest --timeout=1
# .---command stderr------------
# | lit.py: /home/tcwg-buildbot/worker/clang-aarch64-global-isel/llvm/llvm/utils/lit/lit/main.py:72: note: The test suite configuration requested an individual test timeout of 0 seconds but a timeout of 1 seconds was requested on the command line. Forcing timeout to be 1 seconds.
# `-----------------------------
# RUN: at line 11
FileCheck --check-prefix=CHECK-INF < /home/tcwg-buildbot/worker/clang-aarch64-global-isel/stage1/utils/lit/tests/Output/googletest-timeout.py.tmp.cmd.out /home/tcwg-buildbot/worker/clang-aarch64-global-isel/stage1/utils/lit/tests/googletest-timeout.py
# executed command: FileCheck --check-prefix=CHECK-INF /home/tcwg-buildbot/worker/clang-aarch64-global-isel/stage1/utils/lit/tests/googletest-timeout.py
# .---command stderr------------
# | /home/tcwg-buildbot/worker/clang-aarch64-global-isel/stage1/utils/lit/tests/googletest-timeout.py:34:14: error: CHECK-INF: expected string not found in input
# | # CHECK-INF: Timed Out: 1
# |              ^
# | <stdin>:13:29: note: scanning from here
# | Reached timeout of 1 seconds
# |                             ^
# | <stdin>:37:2: note: possible intended match here
# |  Timed Out: 2 (100.00%)
# |  ^
# | 
# | Input file: <stdin>
# | Check file: /home/tcwg-buildbot/worker/clang-aarch64-global-isel/stage1/utils/lit/tests/googletest-timeout.py
# | 
# | -dump-input=help explains the following input dump.
# | 
# | Input was:
# | <<<<<<
# |             .
# |             .
# |             .
# |             8:  
# |             9:  
# |            10: -- 
# |            11: exit: -9 
# |            12: -- 
# |            13: Reached timeout of 1 seconds 
# | check:34'0                                 X error: no match found
# |            14: ******************** 
# | check:34'0     ~~~~~~~~~~~~~~~~~~~~~
# |            15: TIMEOUT: googletest-timeout :: DummySubDir/OneTest.py/1/2 (2 of 2) 
# | check:34'0     ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
# |            16: ******************** TEST 'googletest-timeout :: DummySubDir/OneTest.py/1/2' FAILED ******************** 
# | check:34'0     ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
# |            17: Script(shard): 
# | check:34'0     ~~~~~~~~~~~~~~~
...