diff-merges: let "-m" imply "-p"

Fix long standing inconsistency between -c/--cc that do imply -p on
one side, and -m that did not imply -p on the other side.

Change corresponding test accordingly, as "log -m" output should now
match one from "log -m -p", rather than from just "log".

Change documentation accordingly.

NOTES:

After this patch

  git log -m

produces diffs without need to provide -p as well, that improves both
consistency and usability. It gets even more useful if one sets
"log.diffMerges" configuration variable to "first-parent" to force -m
produce usual diff with respect to first parent only.

This patch, however, does not change behavior when specific diff
format is explicitly provided on the command-line, so that commands
like

  git log -m --raw
  git log -m --stat

are not affected, nor does it change commands where specific diff
format is active by default, such as:

  git diff-tree -m

It's also worth to be noticed that exact historical semantics of -m is
still provided by --diff-merges=separate.

Signed-off-by: Sergey Organov <sorganov@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/diff-options.txt

@@ -49,10 +49,9 @@ ifdef::git-log[]
 --diff-merges=m:::
 -m:::
 	This option makes diff output for merge commits to be shown in
-	the default format. `-m` will produce the output only if `-p`
-	is given as well. The default format could be changed using
+	the default format. The default format could be changed using
 	`log.diffMerges` configuration parameter, which default value
-	is `separate`.
+	is `separate`. `-m` implies `-p`.
 +
 --diff-merges=first-parent:::
 --diff-merges=1:::
@@ -62,7 +61,8 @@ ifdef::git-log[]
 --diff-merges=separate:::
 	This makes merge commits show the full diff with respect to
 	each of the parents. Separate log entry and diff is generated
-	for each parent.
+	for each parent. This is the format that `-m` produced
+	historically.
 +
 --diff-merges=combined:::
 --diff-merges=c:::

diff-merges.c

@@ -107,6 +107,7 @@ int diff_merges_parse_opts(struct rev_info *revs, const char **argv)
 
 	if (!strcmp(arg, "-m")) {
 		set_to_default(revs);
+		revs->merges_imply_patch = 1;
 	} else if (!strcmp(arg, "-c")) {
 		set_combined(revs);
 		revs->merges_imply_patch = 1;

t/t4013-diff-various.sh

@@ -455,8 +455,8 @@ diff-tree --stat --compact-summary initial mode
 diff-tree -R --stat --compact-summary initial mode
 EOF
 
-test_expect_success 'log -m matches pure log' '
-	git log master >result &&
+test_expect_success 'log -m matches log -m -p' '
+	git log -m -p master >result &&
 	process_diffs result >expected &&
 	git log -m >result &&
 	process_diffs result >actual &&