fatal: ambiguous argument "": unknown revision or path not in the working tree.

[nikromen]

This commit that allows configuring the different name of main branch does not have some default value. If no default_branch is specified in dist_git.conf then SRC_BRANCH expands to empty string which leads to this error:

Creating new module branch 'f37' for 'nikromen/test-that-dist-git-builds-from-forks-work-1679990637.4999745/hello'...
fatal: ambiguous argument '': unknown revision or path not in the working tree.
Use '--' to separate paths from revisions, like this:
'git <command> [<revision>...] -- [<file>...]'
fatal: not a valid object name: 'master'
ERROR: Branch nikromen/test-that-dist-git-builds-from-forks-work-1679990637.4999745/hello f37 could not be created
/usr/share/dist-git/mkbranch: line 151: popd: directory stack empty

some check like if default_branch is None then SRC_BRANCH="master" would be handy

[xsuchy]

some check like if default_branch is None then SRC_BRANCH="master" would be handy

I would argue for "main" as a default, but otherwise +1

[FrostyX]

In dist-git-1.16, the default was master, so if the default is going to be main now, we should probably mention it somewhere.