checkpatch.pl: warn on invalid commit id

It can happen that a commit message refers to an invalid commit id,
because the referenced hash changed following a rebase, or simply by
mistake.  Add a check in checkpatch.pl which checks that an hash
referenced by a Fixes tag, or just cited in the commit message, is a valid
commit id.

    $ scripts/checkpatch.pl <<'EOF'
    Subject: [PATCH] test commit

    Sample test commit to test checkpatch.pl
    Commit 1da177e4c3 ("Linux-2.6.12-rc2") really exists,
    commit 0bba044c4c ("tree") is valid but not a commit,
    while commit b4cc0b1c0cca ("unknown") is invalid.

    Fixes: f0cacc14cade ("unknown")
    Fixes: 1da177e4c3 ("Linux-2.6.12-rc2")
    EOF
    WARNING: Unknown commit id '0bba044c4ce7', maybe rebased or not pulled?
    #8:
    commit 0bba044c4c ("tree") is valid but not a commit,

    WARNING: Unknown commit id 'b4cc0b1c0cca', maybe rebased or not pulled?
    #9:
    while commit b4cc0b1c0cca ("unknown") is invalid.

    WARNING: Unknown commit id 'f0cacc14cade', maybe rebased or not pulled?
    #11:
    Fixes: f0cacc14cade ("unknown")

    total: 0 errors, 3 warnings, 4 lines checked

Link: http://lkml.kernel.org/r/20190711001640.13398-1-mcroce@redhat.com
Signed-off-by: Matteo Croce <mcroce@redhat.com>
Cc: Joe Perches <joe@perches.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

scripts/checkpatch.pl

@@ -2900,6 +2900,17 @@ sub process {
 			}
 		}
 
+# check for invalid commit id
+		if ($in_commit_log && $line =~ /(^fixes:|\bcommit)\s+([0-9a-f]{6,40})\b/i) {
+			my $id;
+			my $description;
+			($id, $description) = git_commit_info($2, undef, undef);
+			if (!defined($id)) {
+				WARN("UNKNOWN_COMMIT_ID",
+				     "Unknown commit id '$2', maybe rebased or not pulled?\n" . $herecurr);
+			}
+		}
+
 # ignore non-hunk lines and lines being removed
 		next if (!$hunk_line || $line =~ /^-/);