git log -p -m: document -m and honor --first-parent

git log -p -m is used to show one merge entry per parent, with an
appropriate diff; this can be useful when examining histories where
full set of changes introduced by a merged branch is interesting, not
only the conflicts.

This patch properly documents the -m switch, which has so far been
mentioned only as a fairly special diff-tree flag.

It also makes the code show full patch entry only for the first parent
when --first-parent is used. Thus:

	git log -p -m --first-parent

will show the history from the "main branch perspective", while also
including full diff of changes introduced by other merged in branches.

Signed-off-by: Petr Baudis <pasky@suse.cz>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/diff-generate-patch.txt

@@ -56,7 +56,8 @@ combined diff format
 
 "git-diff-tree", "git-diff-files" and "git-diff" can take '-c' or
 '--cc' option to produce 'combined diff'.  For showing a merge commit
-with "git log -p", this is the default format.
+with "git log -p", this is the default format; you can force showing
+full diff with the '-m' option.
 A 'combined diff' format looks like this:
 
 ------------

Documentation/git-log.txt

@@ -118,6 +118,15 @@ git log master --not --remotes=*/master::
 	Shows all commits that are in local master but not in any remote
 	repository master branches.
 
+git log -p -m --first-parent::
+
+	Shows the history including change diffs, but only from the
+	"main branch" perspective, skipping commits that come from merged
+	branches, and showing full diffs of changes introduced by the merges.
+	This makes sense only when following a strict policy of merging all
+	topic branches when staying on a single integration branch.
+
+
 Discussion
 ----------
 

Documentation/rev-list-options.txt

@@ -108,8 +108,8 @@ options may be given. See linkgit:git-diff-files[1] for more options.
 
 -c::
 
-	This flag changes the way a merge commit is displayed.  It shows
-	the differences from each of the parents to the merge result
+	With this option, diff output for a merge commit
+	shows the differences from each of the parents to the merge result
 	simultaneously instead of showing pairwise diff between a parent
 	and the result one at a time. Furthermore, it lists only files
 	which were modified from all parents.
@@ -121,6 +121,15 @@ options may be given. See linkgit:git-diff-files[1] for more options.
 	the parents have only two variants and the merge result picks
 	one of them without modification.
 
+-m::
+
+	This flag makes the merge commits show the full diff like
+	regular commits; for each merge parent, a separate log entry
+	and diff is generated. An exception is that only diff against
+	the first parent is shown when '--first-parent' option is given;
+	in that case, the output represents the changes the merge
+	brought _into_ the then-current branch.
+
 -r::
 
 	Show recursive diffs.

log-tree.c

@@ -514,6 +514,16 @@ static int log_tree_diff(struct rev_info *opt, struct commit *commit, struct log
 			return 0;
 		else if (opt->combine_merges)
 			return do_diff_combined(opt, commit);
+		else if (opt->first_parent_only) {
+			/*
+			 * Generate merge log entry only for the first
+			 * parent, showing summary diff of the others
+			 * we merged _in_.
+			 */
+			diff_tree_sha1(parents->item->object.sha1, sha1, "", &opt->diffopt);
+			log_tree_diff_flush(opt);
+			return !opt->loginfo;
+		}
 
 		/* If we show individual diffs, show the parent info */
 		log->parent = parents->item;