rebase --keep-base: imply --no-fork-point

Given the name of the option it is confusing if --keep-base actually
changes the base of the branch without --fork-point being explicitly
given on the command line.

The combination of --keep-base with an explicit --fork-point is still
supported even though --fork-point means we do not keep the same base
if the upstream branch has been rewound.  We do this in case anyone is
relying on this behavior which is tested in t3431[1]

[1] https://lore.kernel.org/git/20200715032014.GA10818@generichostname/

Signed-off-by: Phillip Wood <[email protected]>

Documentation/git-rebase.txt

@@ -218,7 +218,7 @@ leave out at most one of A and B, in which case it defaults to HEAD.
 	merge base of `<upstream>` and `<branch>`. Running
 	`git rebase --keep-base <upstream> <branch>` is equivalent to
 	running
-	`git rebase --reapply-cherry-picks --onto <upstream>...<branch> <upstream> <branch>`.
+	`git rebase --reapply-cherry-picks --no-fork-point --onto <upstream>...<branch> <upstream> <branch>`.
 +
 This option is useful in the case where one is developing a feature on
 top of an upstream branch. While the feature is being worked on, the
@@ -452,9 +452,9 @@ When `--fork-point` is active, 'fork_point' will be used instead of
 <branch>` command (see linkgit:git-merge-base[1]).  If 'fork_point'
 ends up being empty, the `<upstream>` will be used as a fallback.
 +
-If `<upstream>` is given on the command line, then the default is
-`--no-fork-point`, otherwise the default is `--fork-point`. See also
-`rebase.forkpoint` in linkgit:git-config[1].
+If `<upstream>` or `--keep-base` is given on the command line, then
+the default is `--no-fork-point`, otherwise the default is
+`--fork-point`. See also `rebase.forkpoint` in linkgit:git-config[1].
 +
 If your branch was based on `<upstream>` but `<upstream>` was rewound and
 your branch contains commits which were dropped, this option can be used

builtin/rebase.c

@@ -1239,6 +1239,12 @@ int cmd_rebase(int argc, const char **argv, const char *prefix)
 			die(_("options '%s' and '%s' cannot be used together"), "--keep-base", "--onto");
 		if (options.root)
 			die(_("options '%s' and '%s' cannot be used together"), "--keep-base", "--root");
+		/*
+		 * --keep-base defaults to --no-fork-point to keep the
+		 * base the same.
+		 */
+		if (options.fork_point < 0)
+			options.fork_point = 0;
 	}
 	/*
 	 * --keep-base defaults to --reapply-cherry-picks as it is confusing if

t/t3431-rebase-fork-point.sh

@@ -50,7 +50,7 @@ test_rebase () {
 
 test_rebase 'G F E D B A'
 test_rebase 'G F D B A' --onto D
-test_rebase 'G F B A' --keep-base
+test_rebase 'G F C B A' --keep-base
 test_rebase 'G F C E D B A' --no-fork-point
 test_rebase 'G F C D B A' --no-fork-point --onto D
 test_rebase 'G F C B A' --no-fork-point --keep-base