branch: add flags and config to inherit tracking

It can be helpful when creating a new branch to use the existing
tracking configuration from the branch point. However, there is
currently not a method to automatically do so.

Teach git-{branch,checkout,switch} an "inherit" argument to the
"--track" option. When this is set, creating a new branch will cause the
tracking configuration to default to the configuration of the branch
point, if set.

For example, if branch "main" tracks "origin/main", and we run
`git checkout --track=inherit -b feature main`, then branch "feature"
will track "origin/main". Thus, `git status` will show us how far
ahead/behind we are from origin, and `git pull` will pull from origin.

This is particularly useful when creating branches across many
submodules, such as with `git submodule foreach ...` (or if running with
a patch such as [1], which we use at $job), as it avoids having to
manually set tracking info for each submodule.

Since we've added an argument to "--track", also add "--track=direct" as
another way to explicitly get the original "--track" behavior ("--track"
without an argument still works as well).

Finally, teach branch.autoSetupMerge a new "inherit" option. When this
is set, "--track=inherit" becomes the default behavior.

[1]: https://lore.kernel.org/git/20180927221603.148025-1-sbeller@google.com/

Signed-off-by: Josh Steadmon <steadmon@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

1 files changed, 16 insertions, 0 deletions

diff --git a/parse-options-cb.c b/parse-options-cb.c
index 3c811e1e4a..d346dbe210 100644
--- a/parse-options-cb.c
+++ b/parse-options-cb.c
@@ -1,5 +1,6 @@
 #include "git-compat-util.h"
 #include "parse-options.h"
+#include "branch.h"
 #include "cache.h"
 #include "commit.h"
 #include "color.h"
@@ -293,3 +294,18 @@ int parse_opt_passthru_argv(const struct option *opt, const char *arg, int unset
 
 	return 0;
 }
+
+int parse_opt_tracking_mode(const struct option *opt, const char *arg, int unset)
+{
+	if (unset)
+		*(enum branch_track *)opt->value = BRANCH_TRACK_NEVER;
+	else if (!arg || !strcmp(arg, "direct"))
+		*(enum branch_track *)opt->value = BRANCH_TRACK_EXPLICIT;
+	else if (!strcmp(arg, "inherit"))
+		*(enum branch_track *)opt->value = BRANCH_TRACK_INHERIT;
+	else
+		return error(_("option `%s' expects \"%s\" or \"%s\""),
+			     "--track", "direct", "inherit");
+
+	return 0;
+}