Makefile: switch to a specific Git branch before update

[Achim Gratz <Stromeko@nexgo.de>]

In a number of recent discussions it transpired that some people expect
to automatically be switched to a specific Git branch when updating.

I've added this as an option to the standard Makefile.  Simply define
GIT_BRANCH (either on the command line or in local.mk) to determine
which branch you want to update.  If this variable is left undefined,
you stay on the current branch as before.

So, this definition in local.mk

GIT_BRANCH = master

would have the update targets operate on the master branch, regardless
of which branch was checked out before the update.  You can still update
a different branch by overriding that variable like

make GIT_BRANCH=my_branch update

in this case or just stay on your checked-out branch using

make GIT_BRANCH= update

if you wish.  If you are working on several branches, I recommend to
leave this definition out of local.mk however and just use the command
line override when necessary.


Regards,
Achim.
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