Re: Getting /usr/src to match specific git hash?

> How cat one track multiple branches with git without keeping entirely 
> separate trees?

There are multiple possibilities here and it is not
clear which you are hoping for.

A) You could be trying to avoid having two separate .git
   copies around, each also with checked-out material.

   (2 separate deep clones.)

B) You could be trying to avoid having 1 .git with
   two directories trees for checkouts.

   (One deep clone with an added worktree, for example.)

C) You could be trying to have one .git copy and only
   one directory tree with only one checkout at a time.
   (In some contexts, this sort of thing might involve
   considerable time and I/O for switching the content
   of the directory tree to be the checkout of a
   different branch. main vs. stable/13 now vs.
   3 years from now might be different.)

   (One deep clone and no additional worktrees.)

D) Sort of like (A) but using 2 Shallow Clones, one per
   branch. (This uses somewhat more space than (C),
   but not a lot more.)

   (2 separate shallow clones in separate directories,
   one for each branch.)

My guess from your wording is that you are after (C).

I presume no local modifications/patches and do not
cover how to handle having such. The command sequences
shown would destroy local changes. It is not clear if
such matches what you are after or not.

I presume below /usr/src/ use for where the clone was
put. It also presumes the fetch status is recent enough
to contain the commit that you want to use.

For building main :

# cd /usr/src
# git switch --discard-changes main
# git fetch freebsd                # if advancing
# git merge --ff-only freebsd/main # if advancing
# . . .

vs. for building stable/13 :

# cd /usr/src
# git switch --discard-changes stable/13
# git fetch freebsd                     # if advancing
# git merge --ff-only freebsd/stable/13 # if advancing
# . . .

In the above the commit is implicit as the
HEAD for the branch named.

There is also being more explicit about the
commit that you want (branch implicit):

# cd /usr/src
# git fetch freebsd # if advancing
# git reset --hard 08b8197a74 # use appropriate hashid
# . . .

This "reset --hard" command produces a "detached HEAD"
notice that you can either ignore --or you can then
create a local branch that uses that HEAD:

# git checkout -b NEW_BRANCH_NAME


For reference, from my context:

# du -sAm /usr/fbsd/main-src/ /usr/fbsd/main-src/.git
2487	/usr/fbsd/main-src/
1379	/usr/fbsd/main-src/.git

So 2487 would estimate (C) as things
are now for main or stable/13 .

2487-1379==1108 for the implicit, primary
worktree that goes with the clone.

A additional worktree tied to the above:

# du -sAm /usr/fbsd/mm-src/
1108	/usr/fbsd/mm-src/

So 2487+1108 == 3595 total using the additional
worktree, i.e., (B).

Just for reference for how much space (D) takes:

# git clone -o freebsd --depth 1 -b main https://git.freebsd.org/src.git /usr/fbsd/main-shallow
Cloning into '/usr/fbsd/main-shallow'...
remote: Enumerating objects: 88695, done.
remote: Counting objects: 100% (88695/88695), done.
remote: Compressing objects: 100% (76042/76042), done.
remote: Total 88695 (delta 18867), reused 51008 (delta 9483), pack-reused 0
Receiving objects: 100% (88695/88695), 258.57 MiB | 9.14 MiB/s, done.
Resolving deltas: 100% (18867/18867), done.
Updating files: 100% (85161/85161), done.

# du -sAm /usr/fbsd/main-shallow/ /usr/fbsd/main-shallow/.git
1378	/usr/fbsd/main-shallow/
271	/usr/fbsd/main-shallow/.git

(The .git is branch specific only.)

So about 2*1378 == 2756 total to also have a separate one for
stable/13 in, say, /usr/fbsd/stable-13-shallow/ .

===
Mark Millard
marklmi at yahoo.com
( dsl-only.net went
away in early 2018-Mar)