convert: Safer handling of $Id$ contraction.

The code to contract $Id:xxxxx$ strings could eat an arbitrary amount
of source text if the terminating $ was lost. It now refuses to
contract $Id:xxxxx$ strings spanning multiple lines.
Signed-off-by: NHenrik Grubbström <grubba@grubba.org>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

convert.c

@@ -425,6 +425,8 @@ static int count_ident(const char *cp, unsigned long size)
 				cnt++;
 				break;
 			}
+			if (ch == '\n')
+				break;
 		}
 	}
 	return cnt;
@@ -455,6 +457,11 @@ static int ident_to_git(const char *path, const char *src, size_t len,
 			dollar = memchr(src + 3, '$', len - 3);
 			if (!dollar)
 				break;
+			if (memchr(src + 3, '\n', dollar - src - 3)) {
+				/* Line break before the next dollar. */
+				continue;
+			}
+
 			memcpy(dst, "Id$", 3);
 			dst += 3;
 			len -= dollar + 1 - src;
@@ -514,6 +521,11 @@ static int ident_to_worktree(const char *path, const char *src, size_t len,
 				break;
 			}
 
+			if (memchr(src + 3, '\n', dollar - src - 3)) {
+				/* Line break before the next dollar. */
+				continue;
+			}
+
 			len -= dollar + 1 - src;
 			src  = dollar + 1;
 		} else {

t/t0021-conversion.sh

@@ -65,17 +65,21 @@ test_expect_success expanded_in_repo '
 		echo "\$Id:NoSpaceAtFront \$"
 		echo "\$Id:NoSpaceAtEitherEnd\$"
 		echo "\$Id: NoTerminatingSymbol"
+		echo "\$Id: Foreign Commit With Spaces \$"
+		echo "\$Id: NoTerminatingSymbolAtEOF"
 	} > expanded-keywords &&
 
 	{
 		echo "File with expanded keywords"
-		echo "\$Id: 4f21723e7b15065df7de95bd46c8ba6fb1818f4c \$"
-		echo "\$Id: 4f21723e7b15065df7de95bd46c8ba6fb1818f4c \$"
-		echo "\$Id: 4f21723e7b15065df7de95bd46c8ba6fb1818f4c \$"
-		echo "\$Id: 4f21723e7b15065df7de95bd46c8ba6fb1818f4c \$"
-		echo "\$Id: 4f21723e7b15065df7de95bd46c8ba6fb1818f4c \$"
-		echo "\$Id: 4f21723e7b15065df7de95bd46c8ba6fb1818f4c \$"
+		echo "\$Id: fd0478f5f1486f3d5177d4c3f6eb2765e8fc56b9 \$"
+		echo "\$Id: fd0478f5f1486f3d5177d4c3f6eb2765e8fc56b9 \$"
+		echo "\$Id: fd0478f5f1486f3d5177d4c3f6eb2765e8fc56b9 \$"
+		echo "\$Id: fd0478f5f1486f3d5177d4c3f6eb2765e8fc56b9 \$"
+		echo "\$Id: fd0478f5f1486f3d5177d4c3f6eb2765e8fc56b9 \$"
+		echo "\$Id: fd0478f5f1486f3d5177d4c3f6eb2765e8fc56b9 \$"
 		echo "\$Id: NoTerminatingSymbol"
+		echo "\$Id: fd0478f5f1486f3d5177d4c3f6eb2765e8fc56b9 \$"
+		echo "\$Id: NoTerminatingSymbolAtEOF"
 	} > expected-output &&
 
 	git add expanded-keywords &&