diff: make default rename detection limit configurable.

A while ago, a rename-detection limit logic was implemented as a
response to this thread:

	http://marc.theaimsgroup.com/?l=git&m=112413080630175

where gitweb was found to be using a lot of time and memory to
detect renames on huge commits.  git-diff family takes -l<num>
flag, and if the number of paths that are rename destination
candidates (i.e. new paths with -M, or modified paths with -C)
are larger than that number, skips rename/copy detection even
when -M or -C is specified on the command line.

This commit makes the rename detection limit easier to use.  You
can have:

	[diff]
		renamelimit = 30

in your .git/config file to specify the default rename detection
limit.  You can override this from the command line; giving 0
means 'unlimited':

	git diff -M -l0

We might want to change the default behaviour, when you do not
have the configuration, to limit it to say 20 paths or so.  This
would also help the diffstat generation after a big 'git pull'.

Signed-off-by: Junio C Hamano <junkio@cox.net>

Makefile

@@ -105,7 +105,7 @@ SCRIPT_PYTHON = \
 # The ones that do not have to link with lcrypto nor lz.
 SIMPLE_PROGRAMS = \
 	git-get-tar-commit-id$X git-mailinfo$X git-mailsplit$X \
-	git-stripspace$X git-var$X git-daemon$X
+	git-stripspace$X git-daemon$X
 
 # ... and all the rest
 PROGRAMS = \
@@ -125,7 +125,7 @@ PROGRAMS = \
 	git-unpack-objects$X git-update-index$X git-update-server-info$X \
 	git-upload-pack$X git-verify-pack$X git-write-tree$X \
 	git-update-ref$X git-symbolic-ref$X git-check-ref-format$X \
-	git-name-rev$X git-pack-redundant$X $(SIMPLE_PROGRAMS)
+	git-name-rev$X git-pack-redundant$X git-var$X $(SIMPLE_PROGRAMS)
 
 # Backward compatibility -- to be removed after 1.0
 PROGRAMS += git-ssh-pull$X git-ssh-push$X

cache.h

@@ -180,6 +180,7 @@ extern void rollback_index_file(struct cache_file *);
 
 extern int trust_executable_bit;
 extern int only_use_symrefs;
+extern int diff_rename_limit_default;
 
 #define MTIME_CHANGED	0x0001
 #define CTIME_CHANGED	0x0002

config.c

@@ -229,6 +229,11 @@ int git_default_config(const char *var, const char *value)
 		return 0;
 	}
 
+	if (!strcmp(var, "diff.renamelimit")) {
+		diff_rename_limit_default = git_config_int(var, value);
+		return 0;
+	}
+
 	/* Add other config variables here.. */
 	return 0;
 }

diff.c

@@ -13,6 +13,8 @@ static const char *diff_opts = "-pu";
 
 static int use_size_cache;
 
+int diff_rename_limit_default = -1;
+
 static char *quote_one(const char *str)
 {
 	int needlen;
@@ -761,9 +763,12 @@ void diff_setup(struct diff_options *options)
 
 int diff_setup_done(struct diff_options *options)
 {
-	if ((options->find_copies_harder || 0 <= options->rename_limit) &&
-	    options->detect_rename != DIFF_DETECT_COPY)
+	if ((options->find_copies_harder &&
+	     options->detect_rename != DIFF_DETECT_COPY) ||
+	    (0 <= options->rename_limit && !options->detect_rename))
 		return -1;
+	if (options->detect_rename && options->rename_limit < 0)
+		options->rename_limit = diff_rename_limit_default;
 	if (options->setup & DIFF_SETUP_USE_CACHE) {
 		if (!active_cache)
 			/* read-cache does not die even when it fails

diffcore-rename.c

@@ -283,7 +283,7 @@ void diffcore_rename(struct diff_options *options)
 			register_rename_src(p->one, 1);
 	}
 	if (rename_dst_nr == 0 ||
-	    (0 <= rename_limit && rename_limit < rename_dst_nr))
+	    (0 < rename_limit && rename_limit < rename_dst_nr))
 		goto cleanup; /* nothing to do */
 
 	/* We really want to cull the candidates list early