xdl_merge(): introduce XDL_MERGE_ZEALOUS_ALNUM

When a merge conflicts, there are often common lines that are not really
common, such as empty lines or lines containing a single curly bracket.

With XDL_MERGE_ZEALOUS_ALNUM, we use the following heuristics: when a
hunk does not contain any letters or digits, it is treated as conflicting.

In other words, a conflict which used to look like this:

	<<<<<<<
					a = 1;
	=======
					output();
	>>>>>>>
				}
			}
		}

	<<<<<<<
		output();
	=======
		b = 1;
	>>>>>>>

will look like this with ZEALOUS_ALNUM:

	<<<<<<<
					a = 1;
				}
			}
		}

		output();
	=======
					output();
				}
			}
		}

		b = 1;
	>>>>>>>

To demonstrate this, git-merge-file has been switched from
XDL_MERGE_ZEALOUS to XDL_MERGE_ZEALOUS_ALNUM.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin-merge-file.c

@@ -46,7 +46,7 @@ int cmd_merge_file(int argc, const char **argv, const char *prefix)
 	}
 
 	ret = xdl_merge(mmfs + 1, mmfs + 0, names[0], mmfs + 2, names[2],
-			&xpp, XDL_MERGE_ZEALOUS, &result);
+			&xpp, XDL_MERGE_ZEALOUS_ALNUM, &result);
 
 	for (i = 0; i < 3; i++)
 		free(mmfs[i].ptr);