merge-recursive: point the user to commit when file would be overwritten.

The commit-before-pull is well accepted in the DVCS community, but is
confusing some new users. This should get them back in the right way when
the problem occurs.

Signed-off-by: Matthieu Moy <Matthieu.Moy@imag.fr>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/config.txt

@@ -126,6 +126,10 @@ advice.*::
 		Directions on how to stage/unstage/add shown in the
 		output of linkgit:git-status[1] and the template shown
 		when writing commit messages. Default: true.
+	commitBeforeMerge::
+		Advice shown when linkgit:git-merge[1] refuses to
+		merge to avoid overwritting local changes.
+		Default: true.
 --
 
 core.fileMode::

advice.c

@@ -2,6 +2,7 @@
 
 int advice_push_nonfastforward = 1;
 int advice_status_hints = 1;
+int advice_commit_before_merge = 1;
 
 static struct {
 	const char *name;
@@ -9,6 +10,7 @@ static struct {
 } advice_config[] = {
 	{ "pushnonfastforward", &advice_push_nonfastforward },
 	{ "statushints", &advice_status_hints },
+	{ "commitbeforemerge", &advice_commit_before_merge },
 };
 
 int git_default_advice_config(const char *var, const char *value)

advice.h

@@ -3,6 +3,7 @@
 
 extern int advice_push_nonfastforward;
 extern int advice_status_hints;
+extern int advice_commit_before_merge;
 
 int git_default_advice_config(const char *var, const char *value);
 

merge-recursive.c

@@ -3,6 +3,7 @@
  * Fredrik Kuivinen.
  * The thieves were Alex Riesen and Johannes Schindelin, in June/July 2006
  */
+#include "advice.h"
 #include "cache.h"
 #include "cache-tree.h"
 #include "commit.h"
@@ -170,7 +171,7 @@ static int git_merge_trees(int index_only,
 	int rc;
 	struct tree_desc t[3];
 	struct unpack_trees_options opts;
-	static const struct unpack_trees_error_msgs msgs = {
+	struct unpack_trees_error_msgs msgs = {
 		/* would_overwrite */
 		"Your local changes to '%s' would be overwritten by merge.  Aborting.",
 		/* not_uptodate_file */
@@ -182,6 +183,11 @@ static int git_merge_trees(int index_only,
 		/* bind_overlap -- will not happen here */
 		NULL,
 	};
+	if (advice_commit_before_merge) {
+		msgs.would_overwrite = msgs.not_uptodate_file =
+			"Your local changes to '%s' would be overwritten by merge.  Aborting.\n"
+			"Please, commit your changes or stash them before you can merge.";
+	}
 
 	memset(&opts, 0, sizeof(opts));
 	if (index_only)