fix(windows): use cmd.exe-compatible syntax for git-log history pass

The git-history popen command used `cd '%s' && git log ... 2>/dev/null`,
which fails on Windows cmd.exe: single quotes are not recognized and
/dev/null is not a valid path, producing "The filename, directory name,
or volume label syntax is incorrect" at the git-history step of a full
index. Switch to `git -C "%s" log ... 2>NUL` (POSIX: 2>/dev/null),
which is valid on both cmd.exe and POSIX shells. Double-quoting the path
is safe because cbm_validate_shell_arg already rejects ", $, backtick,
backslash and the other active metacharacters before the command is built.

Distilled from #325 onto current main (preserves the :%%ct commit-time
field added since the PR branch). Closes #324.

src/pipeline/pass_githistory.c

@@ -225,10 +225,19 @@ static int parse_git_log(const char *repo_path, commit_t **out, int *out_count)
     }
 
     char cmd[CBM_SZ_1K];
+#ifdef _WIN32
+    /* cmd.exe does not recognize single quotes, and '/dev/null' is a POSIX path. */
+    const char *null_dev = "NUL";
+#else
+    const char *null_dev = "/dev/null";
+#endif
+    /* git -C "<path>" works on both cmd.exe and POSIX shells. Double quotes are
+     * safe here because cbm_validate_shell_arg (above) rejects ", $, `, \ and the
+     * other shell metacharacters that would otherwise be active inside them. */
     snprintf(cmd, sizeof(cmd),
-             "cd '%s' && git log --name-only --pretty=format:COMMIT:%%H:%%ct "
-             "--since='1 year ago' --max-count=10000 2>/dev/null",
-             repo_path);
+             "git -C \"%s\" log --name-only --pretty=format:COMMIT:%%H:%%ct "
+             "--since=\"1 year ago\" --max-count=10000 2>%s",
+             repo_path, null_dev);
 
     FILE *fp = cbm_popen(cmd, "r");
     if (!fp) {