Add --squash-history option

[framillien]

Issue: #126

This option is similar to --no-history (avoid bloating the repository size) but keep data. We squash all commits from gh-pages branch filtered on the current commit message.

This allow workflows like keeping commits and data for a released doc and always override nightlies documentations.

[framillien]

Initial commit, openning PR for discussion. Know issues or things to test:

• filter-branch script is a bash script using grep. What happen on Windows machines ?
• filter-branch always write info on stdout, when other ghp-import commands print nothing. Should we mask them ?
• We may want to avoid warning and delay by adding FILTER_BRANCH_SQUELCH_WARNING=1 (currently the caller could define it)

Tested on Linux with:

❯ FILTER_BRANCH_SQUELCH_WARNING=1 ghp-import -a doc
Rewrite a2bccfe1898a5de024d0e1d4110352095c5ad644 (6/6) (0 seconds passed, remaining 0 predicted)
Ref 'refs/heads/gh-pages' was rewritten

❯ ghp-import -a -m "Update latest" doc
WARNING: git-filter-branch has a glut of gotchas generating mangled history
         rewrites.  Hit Ctrl-C before proceeding to abort, then use an
         alternative filtering tool such as 'git filter-repo'
         (https://github.com/newren/git-filter-repo/) instead.  See the
         filter-branch manual page for more details; to squelch this warning,
         set FILTER_BRANCH_SQUELCH_WARNING=1.
Rewrite 07dcc674ba1d249b54623b5e439e2744ff6ad766 (6/6) (0 seconds passed, remaining 0 predicted)
Ref 'refs/heads/gh-pages' was rewritten

[framillien]

FILTER_BRANCH_SQUELCH_WARNING issue is fixed in above commit.

About Windows, this is working with my current git installation: https://gitforwindows.org/ + recommended setup option to add third-party tools in PATH (by default setup option). We may want to add a note in documentation, or just keep it like this.

[framillien]

Playing with GitHub actions, I see Flake8 results, fixed above.

For my Github actions tests, I need to remove Python 2.7 checks, updating Python versions to 3.7 and 3.12. See: https://github.com/framillien/ghp-import/actions/runs/7657957284

[c-w]

Apologies for the delay. Code LGTM. Could you also add your CI fixes to this PR and then I'll merge. Thank you.