which haftara to read on shabbos rosh chodesh av

There is already debate in the gemara (Meg 31) as to which haftarah to recite this shabbos.  Most communities follow the Maharil who writes that we read the haftarah of Yirmiyahu 2, "shimu," the second of the t'lata d'puranusa, and not the haftarah of rosh chodesh. Maharil explains that since there is a remez in the haftarah of "shimu" to rosh chodesh, we cover both bases that way.  The haftarah tells us (2:24)  פֶּ֣רֶה לִמֻּ֣ד מִדְבָּ֗ר... כׇּל־מְבַקְשֶׁ֙יהָ֙ לֹ֣א יִיעָ֔פוּ בְּחׇדְשָׁ֖הּ יִמְצָאֽוּנְהָ׃.  Rashi comments

חדש אחד יש בשנה שהיא ישנה כל החדש ואז היא נלכדת אף אתם חדש אחד הוכן לכם כבר מימות המרגלים שקבעו אבותיכם בכיית חנם בו תלכדו 

Those who disagree with Maharil may not have found this sevara compelling because of you look at Radak on that pasuk you will find two other interpretations of בְּחׇדְשָׁ֖הּ יִמְצָאֽוּנְהָ that have nothing to do with months of the year:

 יש מפרשים החדש שתשלם הריונה, אז היא מלאה וכבדה, ואז יוכלו להשיגה ולמצאה...ויש מפרשים: בחדשה – בחדושה כלומר בכל עת ימצאוה כאלו עתה תחל לרוץ והיא חדשה במרוצה.

 

Shut Divrei Yatziv (183) writes that the debate over which haftarah to read hinges on how seriously you treat rosh chodesh as a holiday.  If you treat it like a yom tov and hold there is a din of simcha (as is mashma from Rambam nedarim 3:9 who lumps it together with shabbos and yom tov and not among days where the issur fasting is only mi'divrei sofrim like chanukah and purim), then you shouldn't read a haftarah of aveilus.  If you treat it like a more minor festival day, not on the same level as yom tov, then the haftarah of "shimu" is appropriate.