Merge branch 'mm/maint-hint-failed-merge'

* mm/maint-hint-failed-merge:
  user-manual: Document that "git merge" doesn't like uncommited changes.
  merge-recursive: point the user to commit when file would be overwritten.

Author: gitster

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::

Documentation/user-manual.txt

@@ -1183,7 +1183,23 @@ $ git merge branchname
 -------------------------------------------------
 
 merges the development in the branch "branchname" into the current
-branch.  If there are conflicts--for example, if the same file is
+branch.
+
+A merge is made by combining the changes made in "branchname" and the
+changes made up to the latest commit in your current branch since
+their histories forked. The work tree is overwritten by the result of
+the merge when this combining is done cleanly, or overwritten by a
+half-merged results when this combining results in conflicts.
+Therefore, if you have uncommitted changes touching the same files as
+the ones impacted by the merge, Git will refuse to proceed. Most of
+the time, you will want to commit your changes before you can merge,
+and if you don't, then linkgit:git-stash[1] can take these changes
+away while you're doing the merge, and reapply them afterwards.
+
+If the changes are independant enough, Git will automatically complete
+the merge and commit the result (or reuse an existing commit in case
+of <<fast-forwards,fast-forward>>, see below). On the other hand,
+if there are conflicts--for example, if the same file is
 modified in two different ways in the remote branch and the local
 branch--then you are warned; the output may look something like this:
 
@@ -1679,7 +1695,7 @@ Sharing development with others
 Getting updates with git pull
 -----------------------------
 
-After you clone a repository and make a few changes of your own, you
+After you clone a repository and commit a few changes of your own, you
 may wish to check the original repository for updates and merge them
 into your own work.
 

advice.c

@@ -2,13 +2,15 @@
 
 int advice_push_nonfastforward = 1;
 int advice_status_hints = 1;
+int advice_commit_before_merge = 1;
 
 static struct {
 	const char *name;
 	int *preference;
 } 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"
@@ -171,7 +172,7 @@ 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 */
@@ -183,6 +184,11 @@ 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)