Fix make git-update: time zones, memcheck, removed directories

[mernst]

make git-update prints the git add/git rm commands for files that a Valgrind merge added or removed. It had three bugs:

1. Time zone. It found files added upstream by the date 1969-12-31, which diff --unidirectional-new-file prints for the epoch only west of UTC. In UTC it prints 1970-01-01, so the target silently found no added files at all.
2. Memcheck. Fjalar's copy of memcheck is valgrind/fjalar/, but the memcheck part printed add .../remove ... (not commands) with valgrind/memcheck/... paths. Its remove branch matched +++ ... 1969-12-31, which never occurs with --unidirectional-new-file.
3. Removed directories. It printed plain git rm for directories that upstream removed, which fails without -r.

This PR is based on master and doesn't depend on any other open PR; it can merge in any order with #96–#101.

Changes

• Match either epoch date.
• The memcheck part uses the same patterns as the coregrind part, maps valgrind-old/memcheck/ to valgrind/fjalar/, and finds removals from Only in valgrind-old/memcheck... lines.
• Use git rm -r.
• Print one command per line, under comment headers (# Memcheck files added upstream; Fjalar may not need all of them, etc.), so the output is easier to review and still runs as a script.
• Doc: "Tell Git about files that were added or removed" describes what the target prints and what to review.

Testing

Synthetic old/new upstream trees with files added, removed, and modified in both coregrind/ and memcheck/, a removed directory in each, and a new file under memcheck/tests/ (which memcheck.patch excludes). Patches were made with the same diff commands as generate-patch-files.sh, in both TZ=UTC and TZ=America/Los_Angeles.

• Old target: in UTC, no git add at all; in Pacific time, add valgrind/memcheck/mc_added.c for memcheck and plain git rm for directories.
• New target: identical, correct output in both time zones. Running it with bash -e in a toy Fjalar repository staged exactly the expected adds and removals, including both removed directories.
• make doc builds.

Real upstream examples are rare: the last memcheck source file added was mc_main_asm.c (3.14.0), and the last removal was in 3.8.0. But the time-zone bug affects every added file.

[coderabbitai]

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📥 Commits

Reviewing files that changed from the base of the PR and between 4263218 and cc052c7.

📒 Files selected for processing: 5

• Makefile
• doc/valgrind-merge.texinfo
• git-update.sh
• plan-upgrade-valgrind.md
• texinfo.dict

• Autofix · Keep fixing CodeRabbit findings and required CI, and resolving merge conflicts

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[mernst]

Superseded by #103.